these are just the source code files
no content or data is included
basically just the engine
these are often outdated compared to source ports.
but they are all the basic building blocks


https://github.com/videogamepreservation/heretic
https://github.com/OpenSourcedGames/Heretic

https://github.com/videogamepreservation/hexen
https://github.com/OpenSourcedGames/Hexen

https://github.com/videogamepreservation/hexen2


among other official idsoftware source codes etc.

enjoy.