Hi !
well our formula was a 4inch diameter pulley on the flywheel and a 06 inch diameter pulley on the alternator, the ratio is 1 to 1.5 which means when alternator is running at 1500rpm the engine would be running at 1500x1.5=2250. you can change the ratios by only changing the pulleys to achieve better load to rpm balance.
The 2SZ has a good low end torque and running it on 2250rpm would create minimum noise. i dont like the neighbours and myself to be disturbed just because of a stupid generator. 
the single pistons usually run at 5000 rpm or above like the Honda 5KVA and have maintainability issues.
rather than using a stabilizer we are using under frequency and over frequency relays to protect the household equipment in any event of failure.
the voltage doesn't effect the equipment a lot, its the frequency which matters, as you have seen fans weening on UPS cause the less the frequency the more the sine-wave is square. it gets closer to DC Square wave and can cause damages.
The mechanical governor is a Centrifuge based mesh plate device, you can search the google for that. It basically tries to contain the desired RPM, here the setting is 2250 on engine.
If the load increases the engine tends to die and the governor senses that and pulls the throttle wire to achieve rpm.
