Even if the pipe had the correct size flange to fit the engine exhaust spigot, and assuming it would bolt up to the KDX frame at all, I doubt that it would run well at all. A 125 pipe is designed for port timing and gas volume/velocity that is totally different than a KDX engine. A 125 revs much higher, has more aggressive porting, probably moves a higher volume of gasses, and is tuned for a much higher rpm range than a KDX. I would bet that it would run awful. Either FMF pipe for a KDX 200 is great. I do think the K 35 pipe is the torque pipe, not the rev pipe, so if the bike feels peaky and soft down low, you might want to have a look at the power-valve. You should have a very torquey power spread, with lots of grunt.