From both hunting and observations, the four parts of a Magnum that seem to me the most difficult to procure in good shape: Gas tank (rusted inside, fine, but holes can be big problem depending on size and location); swing arm (often have found frames without them); seats that haven't rusted completely through (tears, no big deal); forks. Forks are probably the worst, from my experience. Hydraulics work great...until they don't. Then you spend countless hours hunting (unsuccessfully) for replacement parts.
Most of the rest of it is easy. (But don't get a Maxi starter sprocket for the Magnum. Won't fit.)