Well decompiling is only one step in the reverse engineering process. I would recommend taking a look at the Legend of Zelda: Ocarina of Time decompile projects. They reversed engineered the whole thing, which took years and was a team effort.
In the end they got perfectly readable source code, fully documented. And the most amazing thing is, when compiled with the right compiler and right flags, it recreates the original rom perfectly.
I would also recommend a YouTuber called Kaze. He’s been working on Mario 64 for years, re-writing large parts of the engine to get some pretty cool stuff going.