ericw provided a library that is able to extract stuff from the id software ftp shareware.
My common sense gut feeling suggests that one can't release one's own iOS app that is basically a port of one quarter of the content of a game that id/bethesda owns.
Not sure what the shareware legal blurb says, but I'm sure it doesn't allow you to modify the actual content of the shareware game, including modifying it to make it run on iOS.
Custom engines are not "modified shareware content", because they're not based on the binary executable included with the shareware game.
The engine's source code was never part of the shareware package.
That kind of argument is the same thing as saying that the whole full game is under the GPL, just because the source code is. That's nonsense.