Yellowhand, how well do those engines run as free-airs. Do you have any heat issues with them?
No, but I don't leave them running for more than a few minutes if the bike is sitting still...and I wouldn't put one in a kart if the seat blocked the air flow.
Here's another GC with a bunch of hours on it...the fins on the flywheel do keep exhaust heat from melting the recoil starter.