The KDX has it's original 200cc motor in place. While I imagine a kx5 engine in the 125 would indeed be off the hook, I'm not sure how difficult the transplant would be. Putting a 200cc engine into the 125 is pretty easy. I have decided against tearing down my KDX, and am going to keep my eye out for an H series KDX donor bike. I have too much time and money invested in the KDX to feel good about using the motor and parting out the rest. The H series engine will be a bit easier to work with. Head stay in the right place, pipe fitment will be easier, etc..