I had the same problem,compounded by installing electronic ignition.
I found some interesting info on the mg forums re adapting the rev counter.
However, if you are contemplating electronic ignition, it will not work!
The rev counter counts the pulses by utilising a CT. The pulses with electronic ignition cannot be counted.
I sent my unit to JDO instruments
Home page in Yorkshire. I found them very helpful and the service was first class. Other companies can also provide the same service.
Good luck!
David