Hello.
The shafts fit 88-04 kx500.
The gears all fit 88-04 except for these 3
INPUT 2ND,20T
INPUT 3RD,22T
OUTPUT 2ND,29T
That means that 88-89 have the same gearing, 90-04 have the same gearing.
So, you could swap in an 04 transmission in your 88 and you would get the updated gearing