Hey Filmcamera, my advice is just buy a new one. I have gone through a few on my K100 and a fresh start beats fussing with the innards, in my opinion. The price has been going down lately too.
One exception: The odo/trip odo unit can be serviced or replaced, and that can be worth doing. And there are a couple of tricks for coaxing flakey speedos back into service, but mostly that involves chasing gremlins, which is not my thing.
The units are as far as I can tell are interchangeable across all the K100s and K75s. The very early units should be avoided because they lack a modification that allows moisture to vent. The early units have two distinguishing marks: (1) they have two low-fuel lights on the face, and (2) they lack two circular openings on the back, which is the crucial moisture ventilation mod.
Flakey instruments can also be caused by worn/abused connectors at the end of the wiring harness (two oblong blocks of connectors that go into the back of the instrument box). Flood those connectors with Do-Oxit or the like but DO NOT use any kind of grease on them. Each one works through a very delicate spring. I actually spliced a new pair of connectors onto my bike's harness. It was a lot of soldering but it's held up fine. In retrospect I might have been better off swapping the entire harness but for whatever reason I wasn't up for doing that much disassembly that winter. Can't remember why.