What Are The Challenges Of Restoring WWII Military Planes?

I’m a big-time admirer of vintage military aircraft restoration projects. And not just for the personal joys I’ve derived from riding airworthy WWII warbirds such as a B-17 Flying Fortress and B-25 Mitchell AKA PBJ (though that certainly helps). These restoration projects help keep aviation history alive in tangible form, as opposed to just an abstract passage in a textbook or a clip of grainy octogenarian newsreel footage.