AB:

All the hop-up methods that work on the engine in your car will work on the model four stroke engines. The problem is that most have been incorporated in production.

You can get some improvement by cleaning the ports for better air flow, but not a lot. The cam profiles are already about as radical as you want, go wilder and the idle goes to pot.

You can't raise the rpm safely, as the parts aren't designed for it, and your max rpm, if you don't worry about overloading the rest of the engine, is limited by valve float.

It's best, if you want more power from a 4s engine, just to buy a more powerful 4s engine.

Bill.