maybe my suggestions are the wrong kind of advise you're looking for as they are not engine related but,
why dont you try using thinner non-knobby tires.
if your frame allows use thinner larger wheels
regrease and check that the axles spin freely
use a lighter frame.
try taking out the exhaust baffle all together (faster but also louder)
I use road bikes and both my bicycles can make 40+. I use cyclocomputer so not sure how accurate it is. I do keep up with cars though (I usually don't try it makes them mad). its dependent on different variables though, an altitude too high or too big a hill and I dont make those speeds.