Freewheel Sprocket & HD Axle Kit w/Brake - $49.99
4-Stroke Engine Kits
Just to give you a starting point, this is one of the sites that has a fairly good priced and quality kit with the freewheeling gearbox and also a great upgrade is the heavy duty axle kit with the freewheeling sprocket. Some like it, some don't. I like it, but it removes the possibility of the engine and gearbox slowing you down when decelerating. One thing you want to consider is upgrading the wheels if they have the standard BB looking ball bearings to the sealed bearings. A lot of new bikes have these. This is the type of bearings in the axle kit.