As far as I know, there hasn't been major changes to the motor since they actually came out. Since there's really no other 100cc mx bikes (besides the RM100 which is actually a KX anyway), they really have no competition to out-do. In 1994, they switched to inverted forks.. Other than that, I think the major changes were purely cosmetic.
If anyone has anymore information, please chime in. I'm no expert, but this is what I gathered while doing research for my KX100.
Check out this site to see all the different years they came out.
http://www.bikepics.com/kawasaki/kx100/Good Luck!
-Saner