There are specific procedures that must be followed to acquire a
military crash.
First you must write the agency a letter describing the wreck, with all
appropriate information, and ask if they are willing to abandon it for
salvage. You must obtain a letter from the owning agency stating that
they have abandoned the wreck.
Once you have that letter, you may proceed to retrieve the wreck.
This may not be the proper language, but is the gist of the process as
it was explained to me 25 years ago by someone who retrieved a P-51B (?)
off the slopes in Alaska and restored it. ("Moon" Spiller, Versailles Ohio)