I think a RM 250 engine is said to be a good fit. I've seen a few YPVS 350 conversions that look awesome.
Personally I think a ypvs engine is the way to go, I'm not convinced by motocross engines in road bikes. Too much vibration and the wrong internal gearing
people usually go RD250/350 and there is even an adaptor kit you can buy all ready for welding on. Any big singles will crack the headstock eventually - got tried many times.