It depends on how much of a purist you are, if you just want to know the time of day when you are riding, you can get some really nice analogue battery clocks, 35mm clock face, for very little cost, if you are using a 5" speedometer you can self adhesive tape it to glass, covering the overall mileage display leaving everything else visible, I have also used a 3" clock converted to a quartz mechanism, and they look very good, the one thing they struggle with is clunky hands, they just don't seem to get the fine detail that Smiths and Jaeger did, I guess the modern hands are made of plastic and for another (cheaper) market.
Getting stitched up on clocks is not a new (eBay) thing, I was stitched up in the late sixties or early seventies by Joe Shaw who owned Auto Tempo, I think he sort of took over all the Smiths Instruments stock when they packed up in Cricklewood, Joe assured me that he could provide me with an original 3" clock suitable for a Vincent Motorcycle, and he did, sort of, Jaeger! exquisite face and hands, a thing of absolute beauty, all fitted nicely into car dashboard type case, it ticks away beautifully, keeps good time whilst sitting on my desk, never been fitted to a bike, and probably never will be.