The problem with the 83-5 is the rear luggage rack on the bike. That is actually a great feature to have, but makes getting a MX style rear fender harder since it has to be so long to clear the luggage rack. The newer KDXs w/o the luggage rack is pretty easy to convert.
The UFO rear generic enduro fender fits really well on the bike.
Give it up! Get the KX sidepanels installed that I gave you fool with your new $15 Hushpupy silencer! Who cares about the rear fender, I don't cause you are always behind me on the trails!
Ho ho ho, chin pokemon!