OK, bear with me here. I promise it's worth it at the end.
There were three or four pump variations used on these bikes. Any of them will work, but the internal parts are not all interchangeable.
The very earliest ones ('83-'85?) have a male thread on the end of the shaft, with the impeller held on by a nut. These are to be avoided as the male thread tends to break off when trying to free the rusty nut after 25+ years sitting in water. Your bike doesn't have one of these, but you should know to look for and avoid the male thread.
The next variant, '85ish-'90ish has 19 teeth on the big gear, and the impeller is fastened to the shaft with a bolt that fits into female threading in the end of the shaft. You may have this one, or the subsequent one with a bunch more teeth on the big gear. Either pump as a whole will bolt right on to your bike, but the shaft from one will not fit the other (because of the differing gear ratios and diameters). This one will pump more water at idle than the one with more teeth that was fitted through the '90s. I suspect it may cool better.
Now, here's the good part: The 1999-2009 K1200 pump is the same as the '86ish-'90ish pump with the bigger/fewer teeth on the main gear. Don't believe it? Check the part numbers on the fiche. Same housing, same shaft, same gears, same seals. It's the same pump. And, for reasons known only to the eBay gods, they're dirt cheap. The newest part on my '87 K75 is its 2004 K1200 water pump, for which I paid $35.
(As a side note for those who may be ordering the parts for this job themselves: If you open your pump and find a cast iron impeller, you need to order the new stamped impeller and the matching spacer that sits behind it on the shaft. The cast impeller as-is will not fit with the new seals unless a bit of it is machined off. Just get the new one unless you have a mill in your garage.)
