Ian,
Any chronometric instrument can calibrated to suit any machine .All the different dial faces are readily available new,quite cheaply (maybe £15.00 ish) from the specialist restorers. Last time I visited Dave Woods he had seemingly hundreds of different dials on nails on the wall of his workshop and he knows what they all fit.
So dont waste time searching for a correct serial number on an old one from ebay or an autojumble.
A speedo can become a rev counter. A "D" shaped bantam speedo can be built up to be a 5" Vincent rev counter (this will be rather more expensive though) if thats what you require.
Mike
