I think the fork tube diameter is 38mm, which is similar to many more modern mini-cycles, so you should be able to find seals even if Kawasaki does not have them.
Race Tech Cartridge fork emulators would make a huge improvement. Check with them, if you want to spend a little money (under $200).
Kawasaki traditionally puts too soft a spring on the KDX. Cutting a few coils off will stiffen the spring (don't cut too much off or you will get coil bind). If you shorten the spring you will need to add more spacer. Also, raising the oil level will greatly improve bottoming resistance. Try about 4" from the top with the springs out and the forks compressed.