I put together a series hybrid range extender for my electric bicycle, see here:
Series Hybrid with Hubmotor - MotoredBikes.com: Motorized Bicycle Forum
You could do this pretty easily.
A 2 kW brushed permanent magnet electric motor (or a little bigger) with rated voltage at about 80V - 100V at around 10,000 rpm. You want to get about 10 - 20V over your rated battery voltage at the highest torque rpm of the gas motor (below).
A 5hp gas motor, preferably with a governor.
A coupling to tie the output shafts together.
A switch to connect the dc motor output with the batteries.
Adjust the governor rpm to output about 10V over your battery voltage.
When you run it under load, it is self regulating because your amp draw is more than the motor can put out. When you are idling, it may fast charge your batteries but only when you have no load on the electric system. Just make sure your max dc generator output is lower than your controller can handle.