You would have to change the swing arm, the rim, the big expence is the transmission due to the output shaft being not far enough out. If the 300 is that big of a deal, I agree, sell the chopper and get a K-9. Personally I love my Chopper with the 250 on the back. But if your dead set on it, look at spending $4000.00 to $5000.00 for the convertion and that's parts, no labor.