you can get a very simple Trek Incite 4I, it's a 4 function computer for about $15.
Was Fascinated by the shaft-driven bike tho: do you like it? How does it ride? do you have gearing on yours?, does it seem to work for you? Sorry, I'm probably not answering your question but your bike is so fascinating, I wanna know more - from a personal standpoint!
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"