Unless its dirt cheap, like a grand or less, I would not touch it. I have ridden a few of them, from back yard coversions, to very well built conversions, ans they all sucked bad. Vibration cannot be described properly. Parts fall off like you would not believe, and frames crack like crazy, and usually they are not as fast as they should be. Ask anyone, and they will tell you that the stock chassis ( mostly suspension) needs improvement for a 250cc engine.
I would steer clear that thing, but thats just my opinion.