It isn't that hard. I'm currently reverse engineering a old flight simulator game called A-10 Cuba. I had to teach myself X86 Assembly, and understand basic calling convention. Then C++ vtables, struct alignment and struct layout. How-ever you do need this basic level understanding of the core fundamental to help you along when the tools you use IDA, Ghidra that turn the assembly code back into C pseudo code.
So there is a big hurdle to get over in the initial stages but you soon find out that a lot higher code structure/scaffold isn't wiped out by the compiler. For example, the generated assembly code very closely mirrors the C/C++ function boundaries. This enables you to infer the over-all original code structure/layout basically from the call chain, and then you can manually step through and figure out what the original programmer was trying to achieve - abet the order of execution does get messed up by the compiler but it isn't that bad.
In my project with A-10 Cuba, I was successful in reverse engineering its file format, the over-all module layout, engine and rendering engine during my three weeks break. I still have some time to work out the AI logic, and mission design but one builds on another. What do I mean one builds on another? Well when you first start you have no types, not structs. So the first days you think you're making absolutly no progress because you're trying to calculate pointer offsets and structs layouts in IDA. I highly recommend Google Gemini or Claude code to do this heavy lifting because you can get away with a lot by asking it (for this IDA Pseudocode, infer what the struct layout is and tell me what it is doing?).
The first stage of getting those first struct layout is painstaking, then you soon can branch off one strut, or struct pointer to another. This feeds back like a feed-back loop - because programmers are lazy. And you soon have a large part of the struct/code-flow layout figured out.
You then take the structs/code-flow, and pesudo code and then do a re-write in a modern C/C++ compiler until you have a working version.
It would likely be enormously useful in this to have a development wiki behind the games that devs and people that played the games may know the engines, file formats, save file formats, compilers, Langs, etc.
Old devs of the games could enlighten the game preservation community anonymously.
Game dev was a frontier and hardware pushing activity even in the directx era. The magic shift code for ...quake??? Was just the tip of the iceberg.
I played A-10 Cuba a ton on an old Pentium 3. Are you doing this in a repo somewhere?
The plan is to put everything into a repo on github, this includes documentation on the file format, and also the rewrite of the original code in modern C++ and DirectX or Vulkan. I don't see much point in reverse engineering the old rendering engine - I can do it but I've got everything I need right now that I can just rewrite the game inside the browser.
Awesome. Good luck and I hope to see it.