Stumbled across this GX200 upgrade info whilst checking their website for parts.

Removing the governor - this will allow the engine to rev over 3500rpm. There are some caveats to this - mostly that there is a risk of the cast flywheel 'exploding'


GX200 upgrade kit - takes power from 6.5hp to 10hp

It's not a cheap kit, but does look fairly comprehensive and even includes and uprated fuel pump.

To be honest, if you are chasing some bolt on power this kit is probably the way to go.