We're making a patch to run on Windows 10.

Update:

This turned out to be much more complicated than expected.
Grimdoomer has been the main leader of this effort, I've mostly just been tagging along.

We have a semi-working patch at the moment. The game boots on Windows 10, but does not run at the proper speed.
Some things are too fast, some things are too slow, and we're fairly certain the issue is caused by our patch.

There were several issues fixed so far.

#1: Memory Corruption.
The game (for some unknown/questionable reason) was allocating persistent memory directly on the stack.
This data would then get corrupted later because they had just assumed it was safe to do this. While apparently that was something which was safe in Windows XP, it does not work after that.

#2: setjmp & longjmp abuse.
The game implements a coroutine system using setjmp and longjmp.
It seems like this functionality just doesn't work in modern versions of Windows.
So, the solution was to replace the system with our own.
Our system creates a thread for each task (only like 8 or so if I recall) and basically just only leaves one thread active at once, but switches between them when before it would use longjmp.
This seemed like the best way to do it, ie a way which would not rely on undefined behavior.

Currently our issue is that the game's timer does not seem to work properly on modern systems.
This seems to be the final step, since the game appears fully functional, it just doesn't seem to run things at the proper intervals.