I have done some research on this in the past and I guess if someone has nearly a $1000 burning a hole in their pocket, they could put a GX160 crank in a GX200, install an ARC GX200 long rod, a Wiseco 1982P120 piston, and buzz ~.090" off the block. That should get 163cc and a very high rpm-friendly 2.09:1 rod:stroke ratio. The problem is still the valvetrain, and I don't mean just the stuff in the head. Cams and lifters take a thrashing with big lift and RPM. I haven't verified that the above parts are the right ones, so try it at your own risk.
I have a Champion GX200 clone from Costco(from when they were going for $69.99 :biggrin
and a Honda GX140 crank(same stroke as the 160) and I will probably try doing this sometime, but I have some ideas for better holding the valvetrain together.....and for a four valve head, but that's another story. :wink: