The Generac 992 engine in my UltraSource 17500 is set up to run on gasoline only. When I run it on natural gas, the engine runs way too hot! Will advancing this engine's ignition timing allow it to operate cooler on natural gas?  If so, how does one advance the timing? The magnets are built into the flywheel, and the ignition coil pickups are mounted in a fixed position relative to the engine. I do know that Generac uses a variation of this engine for their stationary (standby rather than portable) generators, and THEY ARE designed to run on natural gas. Is it possible to swap my flywheel with one from a natgas powered GT990 engine? If so, would that effectively advance ignition timing? Does anyone at ZE have any knowledge or experience with this issue? Thank you for your kind assistance!

I think that advancing the ignition will increase the combustion temperature, I would suspect that the timing will be the same for either fuel. How did you adapt to natural gas and figure out how to feed the correct mixture? If the engine is too hot it's running too lean/ not enough fuel. With the automatic standby units from 7kw through 22kw all air cooled the timing is fixed and can run natural or LP without any ignition adjustment. You could look in the want adds of this forum and see if someone has a parts unit for sale and compare the flywheels.
Actually, unless advanced too far, more timing will run cooler. 
However, I suspect your problem is more likely caused by a too lean fuel mixture than timing.