that runs on old windows PC

Unfortunately, the only way to make sure your games and their dev environments work on whatever you want is to make your own engine. Unity, Unreal, Godot and all can drop support at any moment, since you’re not in charge, so relying on them will in the long run would, imo, be a bad idea.

AFAIK Godot supports relatively old computers (OpenGL 2). Now, at least.

i dont know what kind of computer you have so im just going to assume windows 7. I would recommend Coppercube, it is very lightweight and it runs on windows xp with the right hardware.