That part at the end of your comment, "...that no one but a single old developer knows how to solve." resonates with me on one of my favorite game franchises from the 80s. Alternate Reality. It was planned to be a 7 or 9 game franchise, all interlocking with The City but only The City and The Dungeon ever got released.

From my research to find the rights holder to that series, which came up a dead end, I did learn from talking with people that worked on the ports to various computers that the original developer kept a lot of details behind the scenes (there are over 10 stats that are not even visible to the player for instance).

To port the game one would need to know the details that only the original developer knew and as far as I know, they are long forgotten at this point as the games were programmed during a period of strong use of post it notes around the monitor. Details that are simply no longer available.

There are many classic games, especially on computers, that are like this which complicates porting even more once an owner is found.