i've had no motor related problems with the two i own. one's got about 1500 miles, the other a lot more than that, and it's heavily modified and i beat the he!! outta it all day long.
the only parts i've really had problems with are chains. they're junk, and like to snap with no warning. and for the record, mine's in perfect alignment...
the way i look at it, is if my motor blew up today, i got my money's worth.