Ok guys, thanks for the advice. I think I might try to go one size smaller. I'm at a 58, so I'll probably try a 56 and see how that goes, without going any smaller. I certainly don't want to seize the engine.
I'm just really frustrated because my acceleration is terrible... if I'm at an intersection, for instance, cars behind me will honk like crazy until I get up to speed. It's frustrating. Once I'm up to speed though, I go along fine at 30+ mph.
I know nothing about mopeds, unfortunately... still learning slowly!