There are some small differences in the 88 motor but 89 up are all almost the same. If I remember correctly the last few years they made them they changed the main bearing size. The early bikes like 85 86 and 87 were all pretty different. I know 85 and 86 had some nasty power. Pain in the ass to start too. The last time I had to start an 85 we pushed a frien that weighed about 240 down a hill in 3rd gear to get it to pop off!!! They had way higher compression in those motors.
I have a friend selling a mint and I mean mint!!! 1985 CR 500. He is asking $2000 but cash talks. That bike is very very fast, but I heard the best CR is an 87? Not sure if that true or not. Im pretty savy on KX stuff considering I have owned them all my life with about 10 years of working in a Kawasaki dealership.