This idea occurred to me this morning as I mentally rebuilt the recumbent tadpole in my mind in the shower this morning. Some homebuilt recumbents use the bottom crank to help route the chain from the boom crank. I was thinking of leaving the bottom crank and derailer intact and use it as a jack shaft shift kit, on a 18/21 speed chain ring that would give you three speeds. on the left side of the crank use a fixed gear to go to the engine, then but a freewheel gear on the end that would go to the boom crank. the only thing you would have to do different is turn your boom crank over so the chain rim is on the left. Just an idea for all you bent builders out there.