I think you'll go electric on the fatty, it looks hard work squeezing an engine in. The Cannondale, fun and games. Sidewinder down by the left of the back wheel? Engine in the frame dip, reduction drive concentric with pedal sprocket? Hang it on a froward extension of the swing arm?
I went over to Redditch on Thursday, helped Doug do a couple of little jobs. He's a one man band, having to work on customer bikes and still settle in to his unit. It's coming, I now know where in the stack my own project is. There was the most glorious old Rudge racing bicycle there, I didn't have anything to take a picture with, but you know the good stuff, might be 100 years old, but get up close and the pure engineering niceness drips off it. This was pedal porn, 5 sizes too big for my legs, but a man can look.