It's wasn't really all that hard. The 125 and 200 motor are pretty close. The first thing you have to do is cut the mounts off the KX frame.
Next you have to modify the swingarm and rear case. The swingarm has to have some material removed from the inside of the swingarm on both side and have the bushing cut down the same amount. It was about 9mm on each side. I have the exact number in my notes. You also have to enlarge the motor mount hole on the rear of the motor. The KDX swingarm bolt is 15,, and the one on the KX is 17mm. That metal is harder then you think it would be.
After doing that you can put the motor in the frame and mount it to the swingarm. It will be automatically aligned with the rear sprocket so no need to worry about that.
With the motor in the fram and bolted to the swingarm you need to space the frame up vertically. I put a thin metal plate under it to space it properly. Then make your motor mounts and bolt them in place. You will also need to either move the frame headstay mount or make a new bracket to mate it up to the headstay on the motor. Also at this time you need to put the pipe on (Pro Circuit fits best) and weld on new pipe mounts. After everything is test fitted and looks good get the mounts welded into place. Then take it apart and paint of powdercoat the frame. I was in a big hurry and painted mine. I will probably take it all apart and get it blasted and coated this winter.
There are a few small items to also deal with. You will need to find a place to mount the electrical parts. I mounted some of mine under the seat. If you decide to build one I can post more pics.
I wonder if a 500 would fit in a KX125 frame?
