First he said a CRF50, then a CRF70. A CRF50 would be legal, as long as you keep your speed down. In AZ, where motorized bicycles are legal, they have a max displacement of 48cc (that's 48cc, not 49cc) I'm riding my 49cc Solex as a motorized bicycle, but if I were ever involved in an accident, and the police or some lawyer started checking, they would find out it is not a legal motorized bicycle. A Solex is fairly well known, and everyone who knows anything about them knows they are 49cc. My guess is a CRF50 is also 49cc.
As far as making it work, anything can be done, but why? There are so many other engines out there that are far more suitable as a bicycle engine. How and where would you mount it, what kind of drive train would you use, and being a 4 speed, how would you shift it? Also, since a CRF50/CRF70 is an MX engine, they make a lot of power for their size, and tend to be really peaky. You would need a strong heavy bicycle to mount it on. Jerry.