Maybe you could put an american to euro bottom bracket adapter in there. That would allow you to use an E-type front derailleur with a triple crank. Put a couple of new chains together. Get a chain tensioner for a motorized bike and bolt it on there. I've also seen an adapter that slides over the axle, is held in place with a set screw, and is threaded to accept a freewheel. I can't remember where I saw that, but it was probably when I was researching motorized bikes. If you run a single gear in back, that would give you a simple 3 speed setup with no welding required. You're a better man than I, good luck.