There have been a few renditions of a FloatingX for M1A1, but non of them looked like they belonged in the actual game of Marathon.
This version matches the interface of the real game, and has the same floaty goodness of FloatingX.
The screenshots are actually from Legend, and the version i captured is slightly out of date, so there may be some very very minor differences.
Finally, The Health and Oxygen bars are misaligned by one pixel... Because of this small margin, I cannot correct it. And yes, I have tried.