Since this is now at a stage where some people might be interested in looking at it:
Marathon Unity
There are no binaries until the thing is much more complete, however it is at the point where it should just run, so for the curious, the instructions are: download the free version of unity, open the project, then open the scene called StartScene, click the big play button in the Unity UI.
Be warned, its got a long way to go. Here is my road map, which will show whats done and what still to go.
-get unity to read map files
-get the basic map geometry drawing as unity game objects
-get unity to read shapes files
-convert all textures to unity materials
-get the textures applying to the level
-convert the collection of hacks that make up marathons platforms into actual platform objects
-make the platforms go up and down
-get basic switches working so doors can open and close
-placeholder fps player controller
-get basic impossible space working
-advanced impossible space (where you can see into both polygons simultaneously)
-lighting
-full control panels (tags, light switches etc)
-platform triggers
-liquids
-get map items spawning (ammo, scenery etc)
-sort out the landscape textures
-get physics file loading into unity
-get the sound file loading
-- at this point the code will require a major cleanup as all the data structures should be in place
-recreate marathons physics
-rebuild the fps controller
-get sounds in the map
-merged map files
-terminals
-in game ui
-weapons/inventory
-enemy ai
-saving/loading games
-out game menus
-probably a bunch of stuff that I've forgotten/not thought of.
*mml files + loading external resources
*scripting - some level of script support should be possible, but i have no idea how practical it will be to support existing lua scripts
*any grarphical extras - realtime lighting, rendering the terminal texts onto the terminals in the game etc
*creation of sloping walls/floors
*ability to connect polygons via floor/ceiling
*some sort of editor to allow for creating all that
There will be bugs. Anyone interested in helping in any way, if you know the files inside and out you may be able to tell me exactly what im doing wrong, otherwise creating the simplest possible map that has the error would go a long way to tracking down the problem. Also, im more than happy to accept code

Finally, impossible space has some limitations (and a bunch of bugs) at the moment (basically it works in my test map, in the maze on Arrival, ye olde 5-D Space and in Ne Cede Malis (which i dont think was meant to have any, but has ended up providing a surprisingly good test case)
Finally finally, the current player has the entirely arbitrary top speed of 7, and the camera is 0.7 wu off the ground, has a bunch of jank when riding platforms and generally doesn't feel much like marathon. Its going to stay like that until the physics files are loaded and working, at which point i will want feedback on weather or not the controller is perfect. Whats there right now is a placeholder. The controls are w-a-s-d + e is action, hold control to get the mouse cursor back.
If all of that was tl;dr, I'll just leave these here...
https://www.acinonyx.me/shtuff/Arrival.jpg
https://www.acinonyx.me/shtuff/ne%20cede%20lights.jpg
https://www.acinonyx.me/shtuff/5d%20space.jpg