I'd wager a plane in a museum is stripped down internally and nothing more than a shell, while a plane on an actively protected military installation is more likely to be complete (or more complete), thus easier restorable.
The SR-71 on display at the Udvar-Hazy branch of the Air and Space Museum located near Dulles Airport in Virginia does not have its engines in the nacelles. The rest of the exterior looks complete, but other internals could very well also be missing and the exterior would still look identical.
The opposite usually. Museums are sticklers for originality. They'll bribe some congressman's mistress to get a meeting so they can get assurances the ATF will behave "favorably" so they can import part of a half parted out anti-aircraft system from Chad ur Uruguay or some shit because that's the only place the obscure gun variant they need for the coaxial machine gun in their tank is still available. Or they'll buy the fuselage of some obscure plane that was only sold to the Indian Coast Guard because it shares all the cockpit instrumentation with some 100 series fighter they're trying to undo the 'nam era mods to and restore to it's 1950s SAC trim.