Recently I've been thinking about "remaking" a PC game from 1999 (Caesar III) for Android. I have no relationship with the original developers, and the studio does not exist any more.
I do not have access to the source code, and will not attempt to decompile the game. I'll write all the code from scratch, basing the mechanism on just what I observe by playing the game.
AFAIK, gameplay is not protectable (by copyright, and there certainly were no software patents back then), so that part should be fine.
However, would there be anything wrong with supplying a tool to convert game graphics, etc, from the original game, to be used in my game? If I don't distribute the files with my game? (requiring the user to own a copy of the original game)
Does anyone have experience doing something like this?