Thought this discussion would be relevant
HyperRocket: 125 mpg, 100+ mph 3-wheel motorcycle | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com
This guy did use a motorcycle engine with good results. Not all motorcycle engines rev to 10K rpms; a lot of the twin v and single jug engines have loads of torque and spin relatively slowly compared to the I-4 crotch rocket motors.
I think a 2 stroke dirt bike engine might even be fun.
Another idea I had, kind of off topic, but somewhat relevant, would be to have 2 motorcycle engines, and only use both when the extra power is needed. While cruising, 1 ought to be plenty.