I have a GC190 Honda engine in my Mokai (mokai.com).
I would like to hop up the engine a bit any suggestions on how to?

go leaner on the jets, and get the carb dialed in so it runs perfect....go one leaner on the plug,............................................. ......... get the motor ported and polished, or step it up and get it bored out, theres a few to think about

or shop around and do some research and put a bigger motor in there....you could prolly fit a 350 or 400 jet ski motor in there....then your talkin about some speed...

Or you can switch to another type of fuel. Maybe Alcohol/Methanol or Nitromethane? Better plug? Better engine?