How do you restrict a 50cc bike?

I think your asking how to restrict the speed of a 50cc. mini bike, you can do this with the throttle cable, adjust the cable to only open the carburetor 1/2 (half) way. If you mean how to UN-restrict the bike, the usual way is with a throttle stop, simply loosen the cinch nut and back off the stop until you achieve full throttle, and retighten the cinch nut.