You will need a different flywheel, kyosho makes different offset flywheels. your best bet is your local hobby shop. I had this problem about 4 years ago & I took the lot down to the shop & we found the flywheel that worked. I dont remember the part number. Try posting in the offroad section you are more likely to find someone else that has been in the same jam as you, only a bit more recently than me.
Serpent Radio Control Models