I actually don't understand the motivation behind these projects. Usually the games for which an open source engine is written for are quite old. Old enough that emulation should be more than sufficient to play these games on something like a Mac with an ARM based SoC. It is also not possible to exactly replicate the game without straight up reverse engineering the game's binary which will open you up for lawsuits. The end result is a very similar game that is merely sharing the same assets but it just isn't the same. If the project was created with the intention of allowing a new experience that is different from the original game then it will be harder than expected because you still need artists, musicians and so on to create more content. Ultimately I think people should focus on making a very good and very faithful clone instead of replicating an old game that still requires you to own the original and proprietary assets.

Of course everything I'm talking about could miss the point of the project and the contributors are just doing this for fun because they like writing code for games but can't create game art on their own.