All

They bolt up.

I installed a 04 pc pipe on a 06 cr125. It had massive snap off idle Very quick but only pulled to 9k & stopped quick & would not rev out. The pipe fit but there’s no way to short shift as quick as the pipe signed off on a track. 03, 04 share the same pipe. But there radically different engines.

It would be worse if it was the other way around but I'd say just grind the port to match the pipe. I had a similar mismatch on my 90 & 96 but maybe 1mm.

These are the only two pics I have of my old 1989 Kx125 (circa 2007). It was pretty much stock. Paid $700 for it with a shot water pump. Rebuilt that and rode it trouble free except for the fact that one of the subport power valves sheered some teeth and ended up rotating in the on position. Basically one valve was open, one closed. When the KIPS kicked it, one closed and the other opened. It basically neutered the top end which was fine by me as it became a learner bike for everyone. It was dead...