IMHO if you are going to rebuild a midget engine and mod the crap out of it to make it work... just find a good 1275 mini engine with a tranny and start there.

Sure it can be done, but really there are more used 1275 mini engines floating around out there, shouldn't be any need to re-engineer a midget engine...

I have done this.

MG Midget 1300 blocks use Cooper S style crankshaft details: the center main bearing is the same as a Cooper S and they use a rod verisimilar to the Cooper S rod with a 1 5/8" big end size.  Still the block is similar to the classic 1300 block with cast in tappets.  The rear main bearing has a machined oil spinner portion that will need to be cut off in transverse use.  The upper portion of the oil spinner and other related in line pieces are not used.

A Cooper S crankshaft will drop right into this block after the rear main has been altered.  A 1300 crankshaft will require the use of different rods, machining down the crank throw big ends from 1 3/4" to 1 5/8" and that is a charming thing in that one can change the stroke when making this change:  Longer, shorter, whatever you prefer.  Since the engine I did this to was a smog era Midget, the pistons had never come near the top of the block, so I stroked the crank just enough to bring them to the deck of the block.  A 1300 crankshaft will require the use of special crankshaft thrust washers, or if you are machining it to use the S size rods, you can have your machinist widen the center main to work with the different size.

There is no need to modify the block's oil inlet.  The O-ring in the transmission will seal.  There is some work required to fit the transverse style dowels into the block, as these are not finished.

Finding an appropriate crankshaft is key to this process.  It is simpler to find a complete 1300 core engine.  However, if the price is right, buy the inline engine, especially if the engine is complete.

where are you? some folks might chime in with engines that they are selling.

Yes it can be done! My 1380 twin cam was a midget motor. But after changing the crank and rear main cap and making a dowel for the oil line from tranny case to motor block. I would not even try to put a 1275 onto a old 850 tranny. I think a better idea would be to get yourself a good 998 A series motor and tranny.

a 1275 midget engine is the same as a 1275 mini engine in dimensions-- they are both A-series engines. The differences stem from the transmission and subsequently the direction the engine is designed to face in the engine bay.

On a midget engine, the transmission bolts to the rear (behind #4 cyl) and is situated north/south for rear wheel drive. On a mini or austin america engine, the transmission bolts underneath and replaces what would have been the transmission pan of the midget engine and is east/west or transverse orientation for front wheel drive

Unfortunately, it's not a direct swap and at minimum you will need to source a 1275 mini crank to put in the midget block. There is a strong chance you will not be able to reuse your existing transmission beneath the 1275 because of clearance issues with the long throw 1275 crank and rods (although you may be able to grind the transmission case to gain the required clearance).

Can it be done? yes, but a Harley motor can also be put in a Prius. Most wouldn't bother. 

Others with more technical knowledge will chime in, but the easy answer is No.

Hi, I am looking for advice on changing out the engine on my Austin Countryman. My vehicle has a 850cc engine and I have a lead on a 1275 cc engine from a MG Midget. Will this engine fit. Don't know the year of the Midget engine, only that it is a 1275 cc.