i 1cylinder 250cc 4 stroke or two?
a 4 stroke should be no problem with a t15 turbo, adequate piping and such, you should see a nice 50% increase given the correct carb tuning. so on a 4bhp engine you should see at least 6bhp, if not more. i have seen turbo lawnmower engines in racing with an increddible 500% increase. (i think, they had about 30bhp)
a 2 stroke would be much harder and you would be better off sticking a bigger engine and a racing pipe and carb on it.
either way, it can be done. look on you tube at turbo lawnmowers or turbo quads etc etc