there's more to it than engine size .. the World Record for a 50cc 2-stroke engine is 140 mph, set back in 1966. Check it out here
Grab this neat little (440K) video from a neighboring page.. MPG movie (for Windows, you could hold the Shift key and click the link and "Save As".. and then view it.)
Wind resistance is the big thing at high speeds .. Double your road speed and the power required to overcome the additional wind resitance is cubed.
So, a bike that uses 5 horsepower to overcome wind resistance at 50 mph would require (5x5x5=) 125 horsepower to overcome the wind at (50x2=) 100 mph. So, streamlining is a vital part of it.
Around here, people talk about going near 45-50 mph or more with a properly set up 70cc kitted bike .. which seems very reasonable.