I think the OEM pieces are still available. If you don't want to spend that much, you can buy new UFO (pronounced OO-fo) from
www.frpoffroad.com and other places.
A KX front fender (or aftermarket replacement) will not have the OEM cable guide on the right side, for the tripmeter / odometer cable. I think the bolt hole patterns match up, so any fender may work. I have't tried it myself, so I am not relaying first hand knowledge.
The KX500 rear fender may work with some amount of work. I think it is not a direct bolt-on, but again, I have not tried it myself.
The KX500 / 1988 - 1989 KX125 / KX250 side panels are the exact same part number, so that explains them fitting the same. :-)
The radiator shrouds are just 1989 - 1994 KDX200 pieces, I think. The KX fuel tanks from that era are different, so their shrouds mount differently.
Congratulations on wanting to put money into your bike. I see a lot of posts on ragged out, older model bikes. It is quite nice to see someone take that level of pride in their machine. Again, congratulations.