Don't have a list of parts for the tacho but this may help.
There are 2 types of tacho internals.
Current sensing type and voltage sensing type.
I am not sure but it appears that the current sensing type has a white needle and the voltage sensing type has a red needle. Other instruments have white or red needles to match.
The only difference that I have found between the GT and Mini S current sensing type is that the redline is at 6500 RPM.
I fixed a GT tacho with a burnt out current sensing wire loop with the innards from a Mini S tacho. The whole internal mechanism is the same, it's just the marking on the RPM scale outer ring that is different.
One other possible difference (can't remember) is that the GT tacho was made in UK and the Mini S tacho was made in OZ?
The current sensing type seems to use the same innards as the Smiths 3" tacho. Google for Smiths tacho or instruments. There is a site that has info about the 3" tacho including a circuit diagram but I don't have the URL. (Didn't save it)
Hope this helps
RonR