I have never personally seen, nor heard, of anyone attempting to modify the Doom source code. It's kind of an insane idea and if anyone ever tries, they deserve to be locked up.
Paintshop Pro (PSP) was widely used for doom graphic editing back then. I remember it having a lens flare effect like this. My money is on PSP or something similar.
This is pretty sweet. I think from a mechanical perspective, you could make your life easier by not going through the frames of their arms swinging. Arms swing opposite to a person's gait. That's not going to be the case when your lower portion is wheels.
I haven't had time to get into this fully, but fyi I soft locked the map at the secret berserk pack by jumping onto the ledge afterwards and then running out to the pyramids