Re: looking for a engine that goes 50mph
Before you start looking for the engine you'd better make sure you have something under you better than a mediocre bike. Even a high end bike is still just that; a bicycle.
You'll have bicycle bearings, brakes, tires and rims. 50mph isn't inconcievable but you're pushing the envelope for safety. Give some serious consideration to what you're asking for.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"