Re: How did this happen?
I think Norm or Tom or some of the more experienced builders on here could give you a better answer, but "mechanical or manufacturing defect" is the first thing that came to mind. If one gear had a weak tooth, that'd break some time, then with the aid of momentum the engine would strip the rest off, I imagine. Or if something were to make your rear wheel seize solid and the chain held, you MIGHT get a stripped gear like that.
I think the important question is: would it be cheaper to buy a new China girl? Or is it safe to fix this one?
"There is nothing wrong with wanting a motorbike that is an extension of your personal taste and fashion sense; if you must ride somewhere, I say do it with style