Hi Hero,
The motor control system that you want should be quite simple to do electronically (famous last words). Off the top of my head, here is a suggested approach:
The main components you will need,
(1) DC power source, a stabilized 12V would be ideal
(2) Slotted opto sensor
(3) OPA2192 dual advanced operational amplifier
(4) TIP41C (NPN) or TIP42C (PNP) medium power transistor
(5) Solenoid powerful enough to operate the engine throttle
(6) Potentiometer
Circuit function
(1) A disc with a hole in it rotates at engine/generator speed
(2) The slotted optocoupler is placed so that the disc rotates in the optocoupler slot. The optocoupler thus produces a pulse for every revolution of the engine/generator
(3) One of the opamps is configured as a pulse counting integrator that inputs pulses from the slotted opto sensor and outputs a DC voltage proportional to engine/generator RPM
(4) The second opamp is configured as a precision amplifier that compares the Voltage output from the pulse counting integrator and a voltage that you set with the potentiometer.
(5) The output from the precision amplifier connects to the base of a medium power transistor that drives the solenoid, which in turn controls the engine throttle.
In this way the engine/generator RPM will be stabilized.
The above is a simple approach, which you asked for, but there are much more sophisticated approaches. The Rolls Royce approach would be to use an Xtal controlled frequency reference and a phase sensitive detector. While this may sound complex. in practice it would not necessarily be.
Another Rolls Royce approach would be to use an Arduino controller to sense the pulses from the opto sensor and drive a stepper motor that controlled the engine throttle. This would be, by far, the simplest and cheapest hardware implementation, but a bit of simple program code would be required. This is pretty much how automobile engine tick-over RPM is stabilized.
The above are just some thoughts.
spec