The numbering of the KL600 goes wonky in 1990.
The U.S. Service Manual applied to A1=1984. The Service Manual Supplement shows that it applies to B-1 through B-5, 1985 through 1989, respectively. So there was a B3=1987. Product year application is on the back cover.
If you got to European Part Websites (UK, Italy, Germany), they list parts for B6=1991, B7=1992, B8=1993 and B9=1994. They account for a production sequence of A1 + B1 through B9. However, on the years they jump over 1990 and associate B6 with 1991.
I don't know why this is either.
Here is a link to one such parts site that shows them skipping 1990 altogether.
For the OP, I have a B1.
Confirm, there are very limited part cross reference opportunities between the 600 and the 650.
The availability of OEM parts through online parts dealers has been extremely good. I've had a couple parts go to back order, but I ultimately got the ones I wanted.
The availability of used parts on places like eBay is equally good from part outs. I've gotten some really good deals buying used instead of new, like I paid $22 for a used cam chain tension that runs $110 (and is still available) on parts websites.
Apparently there was time that aftermarket accessories were available for the KL600. In fact, Acerbis used to make an oversized gas tank for it. However, all the aftermarket stuff like that and tail racks and whatever have long dried up.