Joe, if you’ve disassembled the tach this far, you might as well keep going. What can you loose. You likely have the tach out of the case to see the flaking paint, which is a common problem. To refurbish the interior, use sandpaper and sand all the loose white and blue paint off. Note where the blue paint was. Wipe down and spray bomb the complete interior with a bright flat white. Find some baby blue paint and mask. Then spray the blue area.
With tach out, use a small air nozzle to blow out debris around the circular magnet and the D’Arsonval meter movement. Do not blow air on the hairsprings.
Flaking paint on needle. Use your finger to wipe it off. Use a felt tip pen to blacken or if you are the artist type, a thin coat of flat black lacquer. You WILL change the accuracy of the tach if you paint the orange part of the needle.
If you want an O ring, hit a local hadrware store with an O ring assortment. May not be flat but no one will notice when installed. I forgot the size but can check tomorrow if that is the route you would take.