Porsche had a version of its 356 motor (VW T1 based) that was called a
stationary powerplant and was used for purposes like yours. There
must have been a similar VW motor along the line somewhere. I think
that if the motor is dialed-in (mixture, timing, etc.) it can just
chug along for extended periods. The only question would be just how
much of a drag the generator would put on the motor.