Before you do change it, you should see if you can run it as it is at your local track.

But if you need to change it, then the list is:

Engine - stock is .26 you need .21
Fuel tank - stock is 145 ccm you need 125 ccm.
Front track is too wide. The upper and lower arms can be shortened to legal size, but you will need new cva's. I don't know which will fit though. You can ask nitrohouse about that.