Doing some thinking.
You might adapt a cruise control unit to use as a governor, but I see no evidence of an governor on this thing.
Doing more mental gymnastics the engine is a little under powered.
Typically you would not run a diesel at 3600 rpm to drive a 3600 rpm head, you would ,match a 1800 rpm head too the engine.
You could run it at 3600 rpm but it would be screaming and not running at the best rpm for longevity and fuel economy.
Gear box is kind of pointless if you have a pulley arangement for speed redduction.
Parasite losses in this would be high considering all the gears and belts spinning around.
Direct drive at 1800 rpm the engine might only make enough power to drive a 10,000 watt load ( hard to guess at for sure ).
Not the end of the world but the efficiency would suffer a little.
match the engine a little better ( maybe 2500 rpm is the sweet spot for full power and economy )
For the sake of simplicity I would just shop around for an Onan DJC or milsurp unit ( same ) and have something engineered to work better