Re: looking for a engine that goes 50mph
If speed is what you want you need the THX1138 with the improbability drive.
But really, if you want to hit that speed it's the rest of the bike you need to worry about first. It would have to be rock solid, heavy duty, military grade, re-inforced. But that's just my opinion. There are ways to get there but my best advice would be to read up in this forum's various threads on speed and high performance. See what others have done and go from there.
good luck and wear a helmet
Just a chimp with a gun and an idea we should be free to ride our machines without gettin hassled by the Man.