I have an 85 500. parts are hard to find. Like, I can;t find a piston. They don;t exist/. If it is a watercooled model, it probably has a magnesium clutch cover, and is most likely rotten in/around the water pump. You can probably get a new one, but for around 400 euros. Good place for parts is here
http://www.enduroklassiker.at/index.php?&lang=en or I tried part numbers at munn ktm until I found what was in stock, and ebay is your friend for old busted up crap no better than what is already on your bike :p
If you are a masochist with a boner for old euro trash, sure go ahead, but if I had my time again I;d have gone japanese
hope this helps