Rick, I'm a little surprised nobody has mentioned the head chamber size as an issue. The 12G940 head is about 21.4-21.7cc versus I think 26-ish for the 12G295. Your compression will be waaay too high for a 1098 unless you do some chamber work or put on a cometic gasket (or have some heavily dished pistons). Do the compression ratio calculation before leaping into putting the 1275 head on it.

I don't know where you are knowadays, but if you're anywhere close to SoCal, I have a bunch of 1275 engines in various states of needing rebuild and at different bore sizes some fully assembled some fully disassembled, both A and A+. I even have a bunch of piston sets, reground performance cams, etc. Email me if you're interested and we can work something out.

A stock 12G940 head (with 1.3 intakes) flows about the same as a 295 does. But there are more 940s about, the 295s are getting rare (many have cracked).
A 940 is OK for HP. You will get more torque from a 295 though (so says Graham Russell here in Oz).

 

Any one correct me if I am wrong but wouldn't a 295 head be better than a 940 head on a 1098? 

High compression, nice cam (sw5), cooper free flow exhaust manifold, fresh 295 head, and decent carb setup should go a long way on that motor. 

Look at Technical article by Graham Russell on the small bores.  He generates 90+ hp from 1098s and 70 hp from 998s without rumpty, rump cams.  Quiet, easy power. 

//www.minimania.com/article/1881/Project_Small_Bore__Part_1

//www.minimania.com/article/1882/Project_Small_Bore__Part_2__Cylinder_Head

+.060, MG Metro cam, 12G295, HIF38 on a Howley intake, MG Metro exhaust manifold = 63bhp and 66 torques (and there is more to come when I retime the cam properly).

I am running a 1098 with HIF38, SW5, +40 bore, Duplex gears, LCB and 295 head.  I also run a 3.1 FD in this car. ( head shaved .080" )

I used to auto cross this engine ( with a 3.44 ) and it was quick.  I'm guessing I'm at about 60hp.

I used to have twin 1.25"...which did get a bit more power down low...but find the HIF38 better for the hwy driving I use it for now.  ( still pretty nippy off the line however ).

I would say ...as quick as a stock 1275

I have an MG 1100 and I am thinking about rebuilding the engine but I want a little more power so that I can keep up with other cars in the club (MGBs, canyon runs, etc.)   I have a set of oversized stock pistons and a 940 head for it but I am wondering what can be done to get some extra power out of it such as larger valves, cam, carb changes, etc.   Not sure if it is worth the effort and const versus buying a 1275.  Any help would be appreciated.