Thanks Markiss, I am in the USA. Looking at 25 year old models. Veggivet - That '90 on Bring a Trailer does look really nice.

Like Veggivet mentioned:
https://bringatrailer.com/listing/1990-rover-mini-7/
No sale. Bid to to $15,600.
You still have a chance if that's the direction that you want to go

1990 Japanese Spec Rover Mini with 1330 engine upgrade just went up for auction on BAT. You might want to check it out...

https://bringatrailer.com/listing/1990-rover-mini-7/

Yes, exactly. No disrespect to those that want to carry on the torch of keeping it original. Not saying that you or they cannot enjoy the car in stock form. It's just not my preference.
 
Penfold, being that you say that you want a 90's Mini, not sure where you are located, but are you aware of the importation rules as far as model years? In you don't, in the States it has to be 25 years or older. In Canada it's 15 years or older. 

Happy hunting

I definitely respect the guys who keep their classic cars completely stock. They are essential to the hobby. But I also appreciate restomods. To each their own. I will look more into the swap. I have time on my side, I'm in no hurry. Thanks for the thoughts Markiss

I hunted down a wack of info on the swap prior to pulling the trigger. I went with the D-series. The B-series is too overkill, IMO. But to each their own. Plenty of power in the D-series to have more than enough fun out there and still kick some butt. 
There's also some EV swaps if you want to go that route. 
My advice is to dig into the swap for info (if you haven't already) before you commit just to see if it's for you. There is a bit involved. Whether you do it yourself or have someone else do it. Good to dig up what you need to know. 
Either way, you'll be spending money on fixing and modifying an A-series to get it to a satisfactory level for you, or you can spend money on the swap. Hard to say which will cost more. Depends on the stock Mini engine's condition and what has been done or not done with the previous owners and how far that you want to take it. Hate to sound like a hater towards the heart of such a great vehicle. But, it's old power, and not a great track record of reliability. The purists are angry, I know, I know. 
And I'm not picking on Mini. I'm a resto-mod guy with any classic. Like the classic styling, but modern power, brakes, suspension, drivability, and reliability. 
There's a wealth of knowledge out there. Some of it is on this forum. 'Tuck in' as they say. 

Thanks JM. I appreciate the info. The engine swap does sound achievable. I am not building a car for the drag strip. But I want to be relatively quick on the road...if I want to pass/overtake another car, I want to be able to. And I want to enjoy the car on a twisty road. I am old enough now that I don't care about being the fastest anymore, but I want the power there to enjoy a fun drive.

What are you looking for out of this car? You seem like HP is a concern so im assuming you want it to be quick.

I second the Swap approach. If you have some skills or some help from some people that weld/etc the Vtec swap really isnt expesnive at all. I mean,looking at the prices for a decent Mini,if that is in the budget im sure a vtec swap can be had if you are decent with working on cars. The frame is the main thing,but you dont need the full "10k package" Most of the parts can be used straight off a honda or bought off ebay/slightly modified. A D series is a great motor for a mini as stated my Markiss.

I have a Jap Spec '89 Mini. It was already imported. Happy with it now, but went through a lot of body work to get it to its current state.
Good luck in your search. I'm sure there's some folks here that can help fill in some blanks for you in the process of finding one.
And welcome to the Mini mania. Which is not only a website/forum, but also a serious condition. 
Cheers. 

Thanks all. I can see that I want one. I like British/classic cars. And the possibility of getting a 90's Mini is an attractive proposition...like getting a classic car that's not 50 years old. Deciphering the Japanese used car market is tricky, but I am learning.

Alternatively, it won’t cost nearly as much as a nonMini engine swap to produce a reliable and reasonably powered Mini.

Mini engines are incredibly tough - and reliable if done right. Mine is up to about 150 000 miles since it’s last rebuild. Yes, its getting a bit tired and, yes, it won’t produce 100 hp like it did in its youth but can still manage to pull a 3.1 diff to 5500 rpm (you do the math ;).

Finding the right support is getting more difficult as the real experts check out but there’s still enough inherited wisdom and parts access available to produce a reliable fun Mini at a reasonable cost.

Cheers, Ian

You can modify the stock A-series Mini engine, just like any other engine. There's more than enough parts out there to beef it up. 
You won't find too much out there that's more than the stock range of power, unless it's a special edition, or a car that someone already modified. If you find a true Cooper'S' they can be more exciting than the lower versions. But those can be pricey and sometimes hard to find. Depends what your budget is. 
But for me, I like to turn the key and go. I don't want the car to be down all the time diagnosing it, buying parts, spending money on parts and labour etc. I know that that's what you kind of have to do with any vehicle along the way. But some require more than others. The Mini engines can be needy. They can and will require some investment of funds into it. Sometimes a heavier investment than you'd like.
The Honda swap isn't cheap. Not really insanely expensive. It's an option. Not for everyone. But it will make way more power and is reliable and more modern. Vast array of parts for all kinds of engine choices, and placement. 

Thanks, I appreciate you sharing your experience. Your engine swap sounds like a great solution.

You're not going to be breaking into the amazing power bands in stock form. There weren't that many special editions that had much more power. One of them being the ERA which was turbocharged from factory and making close to 100hp. It will take modifications or engine swap to get more than the 40-70ish stock hp. There are mods to do on the stock A-series engine if you wish. There's a supercharger that you can buy for the A-series as well to get it up there a bit. 
There's also a slew of engine swaps that are available to do with all the necessary hardware and such to make it all happen.
Good thing about Minis is that people have been friggin around with them for decades, they've perfected a lot of scenarios to achieve power gains, parts offerings, how-to planning, etc. 
I'm one that opted for a more reliable approach to power gain and went with a Honda D-sereis engine swap. The A-series stock Mini engines and electricals are too finicky, temperamental, and unreliable for my liking. But that's my opinion and approach. If you have the budget, I'd recommend that. 
Otherwise there are parts and things to do to the stock A-series Mini engine that could get you closer to the 100hp mark. Depends how far you want to take it.

Thanks. So the Japanese market Cooper makes the same HP as other Japanese Mini's (like a Mayfair for example). If I want more horsepower, should I be looking for the high compression engine?

61hp or there about, nothing special that's inside the engine.

I am looking at 90's era (not ERA) Minis in Japan.
How can I tell what horsepower they make?
Does a Cooper with an 1300 SPI with 9.4:1 compression (10.1:1 seems rare even on the Coopers) make more power? What unique engine parts does the Cooper have?
Thanks,
Penfold