a friend of mine put a 1991 250x engine in a 350x frame, it bolted right in, the only thing he had to do is just cut and re-weld the exhaust pipe to clear the frame, he put the 350x engine into the 250x quad, he did this conversion becuase he had hurt himself prity bad on the original 350x and the wife said it has to go or you do sumthing to de-tune it so he swapped engines.
my trikes:
1983 honda atc 200x-k&n filter
2002 yamaha raptor 660r, yoshimura slip on, prm grab bars
1978 chevy nova, 454 v8, bds blower, 150hp shot of nitrous, full roll cage, a real power house.
1991 chevy s10 4x4 350v8.
1986 250sx
YES its fast NO you cant ride it.