Just as an experiment, I pulled an old scrap tachometer apart yesterday, by prying up one edge of the seal ring. Even though that edge of the seal gets somewhat tattered, I was able to refurbish the edge with curved needle-nose pliers - creating a j-shaped edge (in cross section). Since the somewhat mangled lip will end up at the rear of any reassembly, any rough edge will be hidden by the large rubber bushing of the gauge holder.
My only tools were a tiny flathead screwdriver and small curved needle-nose pliers. Oh, and lots of patience.
To reassemble, the inside of the ring would need to be coated with a little silicone caulking, or a new rubber seal (the old ring seal was mangled in the prying-open process).
Instead of prying, sweating and cursing, I could have just cut through the old ring with a dremel tool, and used your brand spanking new ring for reassembly. Much less frustrating and no messy edge - great idea!
On a side note: I couldn't figure out how to remove the needle from the spindle and I'm afraid the pin snapped when I forcefully pried the needle up from the faceplate. Ah well

Edit: added a photo