If the needle won't return to zero and bounces at the slightest vibration the meter return spring(s) are probably broken. They are not practical to repair.
If the tachometer works, but doesn't read correctly that is usually repairable.
I've done about six for myself and club members, but to properly calibrate them can take many hours, or a few minutes. I think that's why folks that did them have quit (or at least no longer advertise) . I never charged to do them for buddies so I didn't have to worry about quesstimating fees.
If all else fails you might find a good used working unit.
DON'T throw away a bad tachometer as a kit to upgrade them with modern components will eventually be available.