I got a 49cc on there. By the way be careful with the real short intakes. They dont sit all the way in the carb tube and you'll have a huge vacuum leak. I had that and it didn't have any power at all. Barely ran at full throttle. Some tape will fix that though.
Hey all-just seeing these replies! Glad I inspired you to try one yourself, Larry! That frame looks nice and beefy like mine. I think you'll be OK. As for the skeptics-I've been riding it to work everyday for 2 weeks. It's been real comfortable with the low height and backrest. The only issue was the torque. It really didn't like low speeds with such small wheels and sprocket. So I pulled the muffler cap and problem solved.
Anyway, no breakdowns, and no...it hasn't collapsed.
Mine is going very well also. I did quite a bit of work on the front and rear mounts to move the motor much lower in the frame. I am going to add a top bar to mount the tank and stiffen it up some. More pictures later of the mods.
This is a fascinating thread! How many of us have a bunch of 20" bicycles that the kids and grandkids have out-grown, just waiting for a project like this? The funny thing is, the China-Girl looks to be "the right size" on these frames!