the RPM that you run the generator at will determine the output frequency, therefore you need to maintain a constant regulated RPM. The trick is that the torque requirements for the generator will change as the electrical load on the generator changes. This requires you to regulate the throttle of the engine to maintain a constant output frequency (i.e generator shaft RPM).

As a rough estimate:

shaft input power(hp) = electrical load(watts) / 768 X 1/ generator efficiency