Here is the full list of all 12 Classic Marathon achievements.
Get a bigger gun
Complete Classic Marathon
Complete Classic Marathon on Total Carnage difficulty
Punch a Drinniol to death
Visit an alien ship
Read the Gheritt White terminal
Complete Colony Ship for Sale, Cheap! without taking any grenade damage
Fly for 7 seconds
Send a message to Earth
Kill the Pfhor cyborg
Fail to notice the secret door
Spend too much time in the lava in Habe Quiddam