Looks like Kawasaki has 3 versions of the KX500 shifter: '82-'91, '92-'93, and '94-'04. Try cross referencing other related shift and drum parts associated with the shift shaft on a newer 2000+ 500. I bet you will find the mating components are the same part #'s (also check the crankcase p/n's). I am willing to bet if you purchase a new shaft, and run a newer year shifter you will be all set.
Kawasaki won't superceede your old shaft p/n because then your older shifter won't work on the new shaft.