To me the thing that kills the top speed of these little engines/bikes is not the engine(although it does help) or the size of the chainring (it too has a effect) but the main thing that kills the top speed is the shape of the biggest thing in the wind stream and that is you and for a lesser part your wheels.
I read somewhere(I think it was bike tech magazine) that the main drag above 15 miles an hour is the wind and if there's something you can do that will cut that drag down you will grow considerably faster.What I did for my bike was to take a pair of handlebars and cut them down. The current bars I'm using are slightly smaller width then my shoulders. I also have my handlebars so that they are about as low as halfway down the drops of my road bike. It may look for peculiar but this little guy really seems to get out of its own way.
I did some coast down tests with my bike without the engine and my friends knew mountain bike. We had the same tires ( 1.5 semi-slick 26) and at the time we weighed about the same. We both started at the top of fairly steep hill with no pedaling just the push off. The way my bike was set up with the closer together handlebars and the lower body position allowed me to slowly pull away from him and by the time I'd gotten to the bottom of the hill I was half a block in front of him and gaining.
My current bike has a smaller rear cog, it's a 36 and a tuned pipe so I guess it is a little hotter than normal
but I noticed the major difference in changing the body position.
The modern mountain bike has a very bad aerodynamic cross-section. If you can change just a small amount it will make a significant difference in your top speed. The fastest human powered vehicles now are going over 80 miles an hour and that is with a lot less horsepower than what we can get out of these little engines.
mikef