If your KX500 is 10-15 years old it's likely that the bronze bushing is worn out....
Almost every KX500 Jason and I have rebuilt over the last 5 years needed a new gear and we use to simply order a new one.
What to do....
If you look closely you can see that this bushing is split.
So, I am looking for a trans gear of the same dimensions ID and OD... The width can be a little wider with no issues...
Just means a little grinding after the bushing is pressed in.