I won't quote any models. But I have bolted many different models of plastic on my '89. Like mentioned above, ALOT of early '90s KX plastic fits on the KX. I don't want to mention any exact years cause I don't want to miss quote you. But I have a KX125/250 fenders, number plate and gas tank.
When I was in Junior High I had a '93 KX125 with a '92 KDX200 motor. It was a fantastic bike. A bike my Dad has regreted letting me sell ever since. He's had three different 200's ('98KDX, '01KTM and now a Gas Gas) trying to find something that matched that peticular bike.
Alittle off topic I know. But I encourage the hell out of you to build a KX125 based 500. I think alot of guys on here are devoted fans of the steel frame. But let me say that once you ride a modern 500 you'll kick yourself for not transitioning sooner. Worth every penny, drop of sweat and broken knuckle