I have had my speedo apart many times upon repairing it, it can be frustrating determining what is causing it to have problems.
I have found after doin it many times though that you can squeeze the top of the speedo housing with large heavy duty hose clamp as opposed to crimping and uncrimping the glass bezel ring. Once you've done it it makes it less of a chore to work on the internals of the speedo (I hated removing that ring). I used a large thin hose clamp I found in my spare parts junk from stuff I've worked on.
I will add that one of my least favorite things to do is reattach the speedo cable upon reinstalling the binnacle, it is just too short to reach and twist it under the binnacle when its sitting on the dash before you bolt it down, then I have to dislocate my wrist to twist it to get it back on. I would rather do the timing belt than the binnacle reinstall in my car, they may not all be this way though, it may be the way the cable is routed. But I've tried numerous routes for the cable.
If you need random parts for your VDO look up "universal VDO speedometer" or "VDO mechanical speedometer" on ebay, it takes some searching because there is so many, but I've bought two entire sets of the type of gauges that are in my Esprit, even the speedo, except its only a 120 mph and not a 180 mph, but it doesn't bother me any, I very seldom even go 80 let alone add 100 to it. I actually like the look of the 120 mph VDO it looks more uniform, the factory 180 gauge has yellow numerals for the KPH, the 120 only has green MPH, it looks cleaner, it confused me the first time I drove it with my new speedo in it, I couldn't figure what was so different.