It sounds like you already know this but if you change over to a newer fork also look at getting the matching triple clamps, front brake caliper, front axle, and wheel.
Depending on what you go with you may or may not need the wheel.
The caliper mounting hole spacing changed a bit over the years.
All of these are KX500:
83-86 used 62mm hole spacing.
87-88 used 89mm hole spacing.
89 used 78mm hole spacing.
90??? probably 78mm but I do not have that info recorded
91-93 used 60.5mm hole spacing.
94-04 used 104mm hole spacing.