I'm wondering if anyone has played with fitting a late model kx250 rear subframe to hang newer style plastics on.
If your just using the swingarm, then the only gain would be the rear fender. The seat probably wouldn't mount to a 500 tank, the 250 uses a perimeter frame so to use the tank and newer shrouds would require modifying the frame. Then when your completed you have a 500 with new style plastic, the suspension is still the same.
I bought a Rocket Science frame kit for my 500, what I ended up with was a 500 that looked cool but didn't handle any better, I sold it and built my first 250/500, much better handling.
I wouldn't want to discourage you, but I would suggest that upgrading to the newer chassis may ultimately make you happier with the bike.