There's a very legitimate question of jurisdiction. The U.S. has no legal authority over the moon, any more than they do venus or mars.
In essence, it would be kind of a dickish thing to do to mess with historical sites on the moon, but the U.S. government has no legal authority over the moon. I'd say something which has been left unattended for 50+ years would qualify as "abandoned", so it's not like theft laws should apply.
There is the issue that if the craft is a U.S.-based craft, then like ships in inte