You certainly can fit a 1275 head on a 998/1100. It has been done many times over.

I once built up a 120 thou over 998 and pocketed the block to fit a 12G940 head.

There are three key things to get right when fitting:
- Ensuring there is adequate clearance for the exhaust valves as they do protrude beyond the edge of the cylinders. Some have got away with a straight swap - A better way is to dummy it all up, measure clearances, and cut a relief pocket in the block. I aimed for 40 thou clearance when the valve was fully open and then a small profile around the valve and out to the head gasket. You need to pick all of your cam/valve train components first. And make sure your pocket doesn't intrude on the top ring (some pistons have the top ring very close to the top of the piston).
- And very closely interwoven is getting your compression ratio correct. A STD 1275 head was 21.4cc (ish) and a STD 998 head 24.5cc (ish) if memory serves me. So on a 998 you could, just by putting on a 1275 head, raise your compression ratio by as much as 1 full number which may not be a bad thing but it needs to be considered.
- And lastly, as Doc said above, you need to fit the head gasket that fits the head ie you need to use a 1275 head gasket.

If done correctly then you will see an increase in engine performance and usually for less than it would cost to get the same performance from a small bore head.

Thanks doc.

In your opinion, would it be worthwhile putting the 202 head on my 998 block?

Cheers

Hi, in fact the 202 and 295 are the best for small bore motors, unfortunately in Europe we are not as experienced as you in Australia for small bore motors ... but we have beautiful engine tuning based on these cylinder heads. the chamber and valve size are good for this type of motor

The 12g240 (1275 head) is too much big for 998 and 1098, as the doc explained

to fit 1275 head on 998 is common in france and UK but it's not efficient for road motor