Re: Which engine?
I've been riding my china girl to work everyday (lol) since august 2011 and it has never taken a dive on me. I did blueprint it like biknut mentioned (opened up the engine, did a quick portjob, smoothed everything out and replaced all the nuts and bolts with ones of higher quality) although I did not do anything as extensive as he has to his. My total cost was probably about $200 including. Although I must point out that I've been building and porting high performance 2 stroke engines for a couple of years now so this kind of work is not beyond me. To someone less experienced this may seem daunting so a china girl might not be the best choice.
From my experience china girls are not so bad but unfortunately you must be willing to open up and work on these little beasts if you want to make sure they won't bail out on you. I do not think that china girls are necessarily "plug and play." But for the mechanically inclined they offer a nice affordable way to motorize your bicycle.
EDIT: Damn I didn't see the date. Sorry for bumping an old thread >.<