The best way to approach this would be to get a small paramotor engine (belt driven with clutch) and convert the propeller drive pully to a brushless electric motor then you could then run this as a hybrid , you could use that motor as a 3phase generator, for charging when you run the engine ( charging would have to be closely controlled or you would overload the engine).
When the engine is stationery, the centrifugal clutch would disconnect and you could use the motor or you would be able to to use them together as well. There would be quite a bit of electronic's and separate controllers involved to control and switch everything
Alternator is around 75% efficient
Motors is around 80 - 90%max efficiency if you are lucky
Current draw at 72V , 13 kW =185amps
Heat, loss, bearings, electronics, batteries, charging, etc will add even more losses
Hope this make sense, this is how they are integrating drone engines/motors. They use the motors to help with take-off.