You mentioned that it "quit".....if it was working for you, pulling it out of the gauge housing and giving it a quick inspection couldn't hurt. However, the 54 has a needle that isn't removable like the earlier models so be extremely careful you don't break it off. Do not remove the large front faceplate.

My guess would be that you have some foreign object between the outer housing and the center section which rotates and indicates the actual speed. TURNING (the cable end)IT BY HAND should have the center section free to rotate. There's a tiny spring that requires special care. If you can't see anything visually, it's going to need to go to a speedo shop.

A puff of air might dislodge whatever is keeping the center section from turning. There is no physical connection between the spinning magnets which also drive the odometer and the rotating center section which moves the needle. Removing the 2 straight slot screws at the rear will not let you remove the assembly as there are two smaller screws behind the faceplate. I managed to break off my needle trying to get the faceplate off, so I know about the hidden screws. Luckily found a used working 54 speedo on the swap meet.

As a "reasonably capable tinkerer" you might just be able to free up the center wheel!

Dave

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