Have fitted sturmey archer hubs (not derailleur) to 2 c5's which worked well, the issue with derailleur gears is the limited amount of space available and the lack of ground clearance with the derailleur hanging down. The hub's I've fitted have been 3 speed and have made the c5 more usable with low start off gear, middle cruising gear and high gear for pedalling anything up to about 20 mph.