I'm not convinced that you could get high 50's/low 60's with pretty much stock everything (especially the crank), there's a reason you don't see stock "builds" like this and it's because they're 99% a figment of imagination looked at through rose colored nostalgia glasses. Even if you were able to build a 50mph ped on mostly stock components it would probably accelerate slow as shit and suck to ride.
All I'm saying is if it was that easy to do everybody would be doing it and there would be almost no aftermarket. So if it's that easy to do let's see one!