Ricky Carmichael "RC4" 2003 Honda CR250R 2-Stroke 1.0

You can now ride in-game with Ricky Carmichael's iconic "Woody" Factory-Honda CR250, which ran in both the 2003 AMA Supercross and AMA Motocross seasons. This is the bike that helped earn RC4 the title of "The G.O.A.T."

Simply drop the .pak file into:
Steam>steamapps>common>MXGP PRO>mxgp4>content>paks

And you're ready to ride.