Seriously though, two things.
My eye sight has been getting worse, and I don't believe a properly tuned china girl, with a correct oil raito will overheat just from continuous running. Mine have run hard for 40 miles or more in very hot weather and I've never experienced an overheating event. It does lose some power when it's very hot, but so does every other air cooled motor.
I think there's a limit though to how much power you can make with a china girl motor before overheating could become a factor. China girls have fairly low compression. I don't think higher than 8:1, and probably more like 7:1. That works real well with the size of the cooling fins. When you start increasing the compression like so many people like to do all bets are off. There's probably not enough surface area to shed much more heat than they already make.
Just for comparison sake, take a look at the difference in the size of cooling fins between a 66cc China Girl motor, and a 49cc Slovenia Girl motor with 10:1 compression.