Rotory Engines

[Speeddog]

But say at 80% throttle and given the weight of the engine itself -- do you know of an engine that is < 20lbs and makes 38hp?  Carb'd so somewhat primitive in that aspect.

At 80% throttle the engine will be less efficient, fuel consumption liters/hour will go down, but power will go down more % than the fuel consumption.

That engine likely has little or no charging system, and no transmission or clutch, maybe a gear or belt reduction to get prop rpm down to acceptable levels.
Likely no cooling system either, at least that would be effective at typical road speed.
Aircraft motors are usually carefully ducted and reliant on consistent high airspeed for cooling.

Dunno what the latest crop of 450 MX/motard engines weigh, likely the closest to that target.