Took the tacho apart to see if I could get it working again as the drive was floating around loosely inside and one of the two screws that secure the dial face had been rolling around on the dial for a while.
Got the drive back in position and applied some Loctite high strength to keep it there and a drop on both dial face screws as well.
Seems to work ok, though I only tried it in the garage giving it a few revs.
Tomorrow I will try and put the screen back on.
Any tips to folding the ring back over?
I used skull's method to get it apart slowly with a bent screwdriver.