I finally ran a Mac 32 cc that I have been modifying. Didn't go all the way to the specs that BME (Keith) posted in the "Don't forget Echo" since I really wasn't trying to get above 8k rpm.

When I started, there was 65tho clearance between the piston and the top of the cylinder without the gasket. I cut 45 thou off the base leaving a clearance of 20 thou, but didn't reshape the top of the piston. I raised the exhaust port to maintain the 105 deg ATDC and cut the piston skirt to give an intake opening of 108 deg ABDC. I did bevel the piston where the ports open 20 thou to keep some of the port area lost when I cut down the cylinder base.
I'm quite pleased with the results.

MA plastic 18 X 6 8400 rpm
Bambuls 18 X 8 8100 rpm
Zinger 18 X 10 7800 rpm