The time at which the source code was lost is irrelevant for decompilation, decompilation uses the binary files. Those are the files that are out there being played right now.
Until recently decompilers tended to produce rough and useless code for the most part, but I’m looking forward to seeing what modern LLMs will bring to decompilation. They could be trained specifically for the task.
You’re welcome to try other methods but LLMs seem to be working best so far.
With a decompiler it should be pretty straightforward to automatically check for “hallucinations,” the compiled code is still right there and you can compare the decompiled logic to the original.