For goodness sakes, you don't need a $1200 governor! I used a simple belt driven centrifugal governor like is used on all kinds of tractors and generators and it works just fine. The engine speed is very steady. There is plenty of rotating mass in the engine, flywheel and generator to give the throttle time to add power if needed.
Here's a governor that should do the job for you with a little adapting. You may have to change the pulley to get it running in it's RPM range.
http://www.surpluscenter.com/item.asp?Item=28-1653&UID=2007100121440082
If the generator is driven at the proper speed it will automatically put out the proper voltage. That's the way they're made. Of course the speed of rotation determines the frequency since they're synchronous generators.