Hold the Command + Option buttons and select "Begin New Game". Then, type the level number and select. This works on all Marathon maps, and Marathon 2 and Marathon Infinity as well.
Accomplish the "11th Clan of the S'pht" achievement to unlock the Marathon logo Gamerpic.
Accomplish the the "VidMaster" achievement to unlock the Vidmaster Gamerpic.
Accomplish the indicated achievement to get the corresponding number of Gamerscore points: