The broken piece is called a clutch shoe of which the Lightning uses a two shoe clutch setup. You can find replacement parts on Redcats homepage or any online website that sells two clutch shoe setups. Below are some links of the clutch shoes you can use:
http://www.redcatracing.com/itemdeta...p?partno=02045 OR
http://www.rcplanet.com/ProductDetai...A4146&CartID=5