By simply monitoring the alternator mains frequency, that saves adding extra speed sensing to the engine and accounts for belt slippage. A compression release solenoid can be held off by the mains voltage, logical ORed with the arduino watchdog...when either dissapears, , belt breakage or the electronic governor fails (watchdog out) the engine will fail safe and stop..
With modern electronics and appliances, mains frequency/engine speed is very non critical, but voltage overshoot is a problem in small systems when heavy loads switch off.
In reality, the existing governor is fine, I wouldnt complicate it.... improve the AVR system, thats far more important..