As far as I'm aware, the reason that there are copyright issues with old aircraft stems from legislation which is in place to limit liability.
The reasoning goes something like this: Aeroplane enthusiast X wants to build a replica of a Flying Fortress (which would be damned impressive if he could manage it) however, if he builds it and subsequently crashes it and kills himself, his dependents could (in theory) have a legal claim on compensation from Boeing (since it would have been their design which would have resulted in X buying the farm).
Silly I know, but this is the legal world in which business operates and they don't like loose ends, no matter how far fetched any scenario which could derive from it might be. And so, silly as it might seem, and indeed often is, much of this legislation wraps things up so tight that even reproducing one in a simulation gets the big no no. This is one of the reasons why vintage aircraft are worth such a lot of money and probably why you invariably see Messershmitt 108 Taifuns, and the Spanish copies playing the part of Messerschmitt bf 109s in naff war movies
