With bicycle-engines being out of stock and the last I heard not expecting any 4-stroke product until the end of summer, you may be out of luck? You might try the Folks at bikeberry.com They bought the Inventory from Spooky Tooth earlier this spring and I know there were some 4G Kits in the Inventory, but they seem to have already sold out of them. Maybe they have some spare Parts?
ocscully