The Honda CRF50 motor will bolt straight in...Its what pit bike engines are based one (Dhz, pister, motovert, pit pro, atomik, dream, mso, motoworks just to name a few)
But the 2 smokers wont i think it would be cool to put a 2 stroke in one but maybe a 65 or 85...Heard a few people talk about doing it and i think there is one on planetminis.com