Moped Engine Swap....seriously
how about a turbo suzuki 4stroke in a YSR? 
Optimus_Prime:
luckily, its not hard to fix your problem realisticly...
they put limiters and restrictions in the bikes already.. if it has a speed limiter, you can disable that... most likely it has a large baffle/restrictor in the exhaust.. ditch the muffler and replace it with a open dirtbike one.. (or other compact muffler.)
if you want to start getting into it, replace the tiny carb with a bigger one off of a dirtbike...
oh, and theres always nitrous (no, im not kidding... you can do it easily.. lol)
Optimus_Prime:
luckily, its not hard to fix your problem realisticly...
they put limiters and restrictions in the bikes already.. if it has a speed limiter, you can disable that... most likely it has a large baffle/restrictor in the exhaust.. ditch the muffler and replace it with a open dirtbike one.. (or other compact muffler.)
if you want to start getting into it, replace the tiny carb with a bigger one off of a dirtbike...
oh, and theres always nitrous (no, im not kidding... you can do it easily.. lol)
You could also go this route 
. You know what they say: If it is good enough for Leno..................... http://www.marineturbine.com/motorsports.asp 
.
David
. You know what they say: If it is good enough for Leno..................... http://www.marineturbine.com/motorsports.asp .David
