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DaveR: I think it will depend more on IF it is repairable. My old Mercruiser rev counter could not be opened (and, believe me, I really tried) so I replaced it with a Volvo model, which also cannot be opened. I think you may need to consider a replacement.
As Marshman says it is just as likely to be the input to the tacho that has gone AWOL as the tacho itself. What checks have you done to confirm it is the instrument? Ideally you need a signal generator and/or an oscilloscope to make sure but often as not just wiggling the connectors, particularly at the alternator end if it is one of those types, can spring the things back to life.