Zdoom/Gzdoom mod compliant GPL engine:
https://github.com/nashmuhandes/GZDoom-GPL

1:1 improved asset replacement, mod compliant:
https://freedoom.github.io/

pol.wad:
http://www.wad-archive.com/wad/2781793ecb5b60b0dc40ba37c1572c05

Engine for original dooms, heretic, hexen and strife:
https://sourceforge.net/projects/vavoom/