As I move forward with the service and maintenance schedule on my recently purchased 40 year old K100C I looked pensively at the water pump cover and wondered what nightmare awaited behind it. Has the impeller even been upgraded? If I have to pull the shaft will it be pitted beyond use? If I leave it alone is a pump failure impending on the first good ride? So off with the cover it was. No, the impeller upgrade had not been done, so removing the pump it was. 
Having done this job a number of times I locked the shaft with an Allen key and prepared to go into battle with the rusty nut, only to have it fall off with the tip of the shaft snapped off and still inside the nut. The shaft was very badly corroded where it passes through the seal which is not surprising given its age but it’s toast anyway so a new one in needed. The pictures tell the story however, with the condition of the seal well past it’s use by date, and the surface of the impeller blades so rusty they have lightly scoured the face of the pump but I’m thinking a polish with some 0000 wire wool and it should be fine though. Any thoughts on this?

Wow!  Somebody used well water in the cooling system and didn't bother changing the coolant very often.

If you think that's bad, the Mk.1 impeller I took off LFB was pretty much a solid lump of rust. Wish I kept it but....

Change the impeller for the later (Mk.2) version. It's a different design of shaft (stronger), plus the impeller part is a lot lighter and according to Motorworks, flows a lot more coolant.

If I can get some clear coat to stick to the rust, I’m tempted to use it as coffee table art although I’d likely be shut down by my better half on that idea! Dai, the reason for the cover removal in the first place was to see if the impeller upgrade had been done.

i have seen this but only on bikes/cars which have been left standing for some years (british cars)  , replace it and refill with the correct water mix and use the bike .