There is a gear set on Ebay from a 2009. I think it will work in the older bikes but would like another member to confirm.
http://www.ebay.com/itm/GAS-GAS-EC30...NXohMK&vxp=mtr
You should also check with this member too.
http://www.gasgasrider.org/forum/showthread.php?t=22228