I've taken LZDoom which is a flavor of ZDoom that runs on older hardware
and packaged it with assets hosted by the Freedoom project for easy install on older versions of Windows Specifically ReactOS.
ZDoom is an open source port of the Doom engine and can be downloaded from here.
https://www.zdoom.org/
The Freedoom project aims to create a complete free content first person shooter game. But Freedoom by itself is just the raw material for a game.