In the in-game art, the ramscoop (assuming that's what it is) is on the side of the ship facing the camera, so whatever the propulsion system is (which would be opposite the ramscoop) would just have been out of sight.
I wonder what kind of propulsion the Marathon would use. I think ramjets are supposed to be basically Orion-type drives, no? Sucking up hydrogen with the ramscoop and fusing it for propulsion out the back?
I'm not sure what an Orion thruster would look like.