A very cost effective way
is to change the intake and exhaust timing of the valves. This is done by changing the gears on the camshafts. Another possibility is to polish all channels on the cylinder head or at least to remove the burrs from the production. The aim here is to make the gas column less turbulent, which results in a cleaner, smoother and thus faster exchange of gases (both on the intake and exhaust side), especially during load changes. With more displacement, you can also breathe more life into the KLX. But this entails further measures. The injection must then also be adjusted, so the ECU must be changed. Modifications can also be made to the air box, which is not part of the actual engine, and the same applies to the exhaust.

The full program
is no longer limited to the engine, rather everything from the air box to the exhaust is possible. I'll list them as best as I can. Complete reworking of the cylinder head with larger valves, steeper camshafts, big bore cylinder kit, longer crankshaft stroke, larger throttle body, different ECU, racing exhaust, high-flow air filter. Here a measure almost always requires changes to other components.