I ride outdoor motocross or at least plan to!
Stock! Don't touch a thing! The KX has a great motor. If you want to be competative in whatever class it is you're riding in, the stock motor is NEVER going to let you down or otherwise be a weakness.
#1 Suspension: Sounds like you've taken care of that already
#2 Ergonomics: You have to feel absolutely comfortable on your machine to ride it with 100% of your ability. The KX has a very cramped feel for my 6'2" build. But things any rider could benefit from are wider footpegs, extend the clutch actuating arm and I liked using a '89 KX250 gas tank. It was a little slimer in the midsection.
#3 Gearing: This is the only aspect of power you need to mess with. IMO the KX (or CR even) comes geared too tall for todays MX tracks. I'd use a 49t at a minimum, probably settle on a 50. This would be far cheaper and more beneficial than any motor mods you could waste your time/money on.
Honestly, unless you don't mind looking like a dork put putting around on a 500. Leave the motor alone and fix what you must to allow you to ride at the best of YOUR abilities. You are not a faster rider than that motor is capable of going.
