Why would it need to rev high? Use gearing that keeps the engine in the ideal peak of the powerband. Boring the engine won't double the horsepower but if you have the tools it is easy to do and will help. If it has to be a single speed, the rear would have to be around 15 tooth.
If a bicycle can reach 80, I'm sure these engines could also
80 could never be reached by just slapping an engine onto a rusty old barn find but with a custom built bike, a properly built engine with a few mods and body work, I can see getting pretty fast.