you could use a bicycle style free wheel sprocket, if you could find one strong enough. the way traxxas ( and i think comet) makes theirs is, sort of like the recoil starters in our engines. when the rpm get high enough a pawl flicks out and grabs. just like a clutch. unlike most clutches, when the rpm gets high enough, the other gear locks in unlocking the previous gear. when rpm slows enough, the spring on the pawl pulls the pawl back, thus reverting to the lower gear ratio. the problem you may have with a standard free wheel is, if the pawls don't retract. there are what, three to four pawls for most FWs? in the traxxas setup, there is one. in a recoil starter there are one, two and three depending on manufacturer. the standard FWs are pretty wide as well. meaning that you couldn't run them right on the wheel hub if you wanted. with the length involved, you would almost certainly need a js, and a bearinged bracket to stabilize that end of the js. if using ten speed or higher sprockets, the tranny might be able to only add six inches to the width of the engine. you would need, preferably, three multisprocket (hyphen busted) FWs, deciding which gear you would want to keep. you would need a piece of tubing threaded for the FW assemblies and keyed for your js. engine side would need a sprocket stack from at least an 18 speed. spacing these to where they have enough strength will be the difficult part. you may be able to use a mower's blade holder to attach the sprockets, maybe not. it will be considerable work.
