There have been Keen3 mods made. That one where microscopic Norp the Yorp goes inside the body. I was never able to finish the thing (way too hard for my tastes), but it offered a unique way of finishing each level... instead of a door, you had to shoot a white blood cell. That white blood cell had the properties of the heart in the Mangling Machine, and that ended the level.
Plus, his levels also used the leg and arm sprites, so I would suspect that the Mangling Machine level can be manipulated quite a bit.
Discussion and analysis of graphics, story, levels, and so on.