I had a '91 KDX 250 (USA spec). The fork springs were WAY too soft. Too compensate for the soft springs, Kawasaki uses too much high speed compression damping. So you get a bike that is too soft on whoops and jumps, but still too harsh on rocks and roots. Worst of all worlds.
You can make your stock forks much better with a revalve and stiffer springs. For mine, I used Race Tech gold valves. Race Tech sold me the parts and then it was a do-it-yourself operation. Or you can pay a pro to do a revalve.
I think most current KX forks are far superior, but you may need softer springs and they might also benefit from re-valve. Not sure what years are the easiest to drop in to you KDX, but I know it has been done before.