Did you get anywhere with this job?
I’ve just bought a ’95 ETZ 251 with a tach that has a badly cracked lens. On close inspection it looks like the base of the rev counter is bonded or ultrasonically welded into the housing that contains the lens.
I’ve seen rev counter housings for sale on their own, suggesting the unit can be dismantled, but I can’t see a way of doing this short of careful sawing of the old housing. As the unite currently functions ok, this seems a little drastic!
Any learning you managed if you managed the replacement would be interesting to know!