The source code for DOOM III
has been released and you can find it on GitHub
. Now, there aren't any assets to go with it, but you can poke through and see how the game was made to run. If you're a developer, there's a lot of really good information here and you can take the code to do with what you will.
If I had the time, I'd pour over it to see how things get done and learn a bit. I wish more game developers released their code though as this is a great way for up and coming developers to see the inner workings of a published game.
DOOM III was released in August 3, 2004 so it might be another seven years before we see the release of the Rage