Can you use a 49cc pull start for a 50cc desert racer dirt bike?

Yes as long as the mounting and electrical are the same. However, there will be a slower starting off and it will make your bike non- street legal in some states. If you want to allow your bike to be street legal and not have to worry about cops apprehending or taking your bike away for 1cc of difference for riding on a local neighborhood street or on the highway.