The main problem are spare parts in general as KTM went in bankrupty in the beginning 90's when I remember right.
So any parts and expecially plastic's, engine case (the magnesium case on the clutch and waterpump side corrode very quick),
clutch parts, even piston's are a problem.
The "oldie but goldie" section of KTMTalk is really a good ressource.
Anyway good luck to your rebuild, these bikes still running and complete
will be rare bird's in a couple of years I believe.