There are, actually, severl open-source 'ports'

ranging from trying to mimic the original engine (aleph one), to rewriting it to use the Unreal Tournemant engine (Marathon Ressurection)

I you feel like following a multitude of links, http://marathon.bungie.org/ (org, not com) has all this information on the site. The parent site for that (for the curious) is http://www.bungie.org this also has links for myth and Halo (me want halo!)

http://resurrection.bungie.org/# is marathon resurrection
http://source.bungie.org/ is aleph one

yes, I like marathon...I played the beta of M1 when it was leaked to the internet in...'94, I believe...