hi 5-7heaven, I am working on a biuld like yours and am planning on using a mixture of motorbike and bicycle components to a homemade jackshaft, dont have a picture so will explain layout. I am going to use strimmer engine with centrifugal clutch and make a spacer to weld to clutch to take a 14 tooth sprocket of a 125cc motorbike gearbox machined to accept bicycle chain, then on left side of bike I am going to use a 34 tooth minimoto sprocket machined to take right chain to back wheel with a bmx freewheel onoutside, not sure no of teeth, to go to original pedals, the rear cassette will remain untouched