To my knowledge reverse engineering is legal (depends on jurisdiction of course). As is emulation. Which means there should be no legal issues with getting the source code of a binary executable, and then modifying it to work on another system.
A bit tangential, but you can also try to look at this logically. The number of instructions in the binary is limited. So the probability to get "substantially similar" binaries is higher than, say, with a book (for the same architecture of course). Plus as I mentioned, the videogame is not just a binary, it's also some assets with which the binary works. And we are not touching the assets since they are mostly independent of the architecture.
A bit tangential, but you can also try to look at this logically. The number of instructions in the binary is limited. So the probability to get "substantially similar" binaries is higher than, say, with a book (for the same architecture of course). Plus as I mentioned, the videogame is not just a binary, it's also some assets with which the binary works. And we are not touching the assets since they are mostly independent of the architecture.