If you rev counter has broken down due to mechanical reasons you may consider modifying to electronic solutions. When I modified my MG VA to a modern engine I lost the dynamo drive for my tacho so had to look for other ways of getting the required info to let my rev counter register. The inside mechanism was removed (and stored for later use) and replaced by a modern electronic rev counter mechanism. This is how it looks:
The set up is insulated in such a way that minus or positive to earth does not matter. The three wires are connected as follows: red to positive 12V, blue to earth 12 V, yellow in parallel to the CB side of the coil. Modification carried out by: Van Putten & Son, Clocks and Instruments from Rotterdam, Holland.