If your clutch is sticky it's not necessarily worn out.
I've dealt with the sticky clutch syndrome (SCS) on a few occasions when buying a new/old bike.
One easy thing to do is to change the oil.
I like the Amsoil but there are many many good brands out there (Let the debates begin)
Likely the bike needs a oil change anyway and it's a lot cheaper then buying a new clutch.
Clutch pulls on KX500's are more severe then on most other bikes.
There are several things you can do.
A new cable is a good start...
Where the metal sleeve makes the bend, straighten the bend out a bit and route the cable so that it makes as few and as large of a bend as possible.
Next, get one of these..
I'm out but Adam Millar might have some:
https://www.facebook.com/MRE500/This will make your clutch pull 20% easier then stock..
For a piston... (Another Hot topic)
For a stock build stay away from a Namura, but just about any other is fine.
For a modified engine Wossner.
Personally I run a Wossner in everything I can regardless of the mod level.
Right now they are hard to get but the manufacture is working hard to get more.
I have no extra plastics but be patient and check eBay frequently..