Initially I put on a Stroker head pipe with the stock muffler & White Brothers Hot Tip exhaust insert on my '99 KLX 300. That made a remarkable difference in the power. I later put on the Stroker silencer for even better performance and to shave off a little more weight.
But, even more important if you have not already replaced is the Idler Gear.
The stock idler gear in the KLX 300 has a Rockwell hardness that makes them brittle. They also have excessive play on the output shaft. The result can be serious motor damage if the teeth break off. The Stroker idler gear has a less Rockwell hardness, which takes all the brittleness out of it, and the tolerance is tighter on the shaft. The tighter tolerance will not allow the gear to wobble side to side while kicking the motor.