You are no doubt right as much as I like the thought of the drive coming from the cylinder it will be simpler and a more robust option to go with the drive from the firebox.
Yesterday my cordless 18 volt drill/driver went pop or at least the pcb did, knowing I would have no chance of getting a new board off I went and bought myself a new drill.
This morning I stripped down the old drill to see if anything was worth saving in the maybe come in useful box. The thing that did draw my attention was the gearbox parts (picture below) I immediately thought differential.
Now I dont know yet if I can make anything of the parts I need to put pen to paper but it would be good if I can, I do like the idea of having a differential on the engine. I know its not essential and a single wheel drive works well enough but a differential would be good to have.