Many thanks for your replies;
I think I should point out that I have been racing for over ten years, mainly japanese 4's, but have been 'developing' the Rotax just to be different!
A high compression piston was an idea but no-one appears to make one for this engine and as Steve H says will probably divide the chamber, (twin plugs?), the stroke is around 81mm which is the longer throw of the singles so a shorter stroke, 79.?, from the 500cc, is a possibility.
At the moment we have kept peak power and torque to around 6600 rpm which seems to be fairly stress free for this motor, we achieved this by lengthening the intake and using a 45mm carb.
The other idea I've had would be to fit thinner copper head and base gaskets to increase compression, the same problems of timing and valve clearance will still occur but no metal will have been removed which can't be replaced.
Another major concern would be the increased engine temp that high compression will create, this being an aircooled motor.
johnr, I'm intrigued, if it was your money what would you do?
many thanks