I haven't done it with my '81 R65, but typically for sealed units like this, you have to very carefully remove the bezel and the glass to get at the guts.
If it's like the units on my older bikes (R60/2, R51/3), you'll have to go around the outside edge of the bezel with a fine screwdriver and gently pry the edge out just enough to eventually be able to "scooch" it off the body. If you're really careful and patient, you may be able to do this and then put it back together later. If not, you're probably out of luck and will have to send it to a speedo house, because none I've talked to will sell parts.
(The speedo shops just pry and remove the bezel, and have a jig to reinstall a new bezel. The bezel is made from a springy steel that clamps back shut around the body and - hopefully - creates a tight seal with the glass and body. This is what you have to maintain when you try to remove and reuse the bezel.)
