: Links, anyone? I'd appreciate it very much.
M1A1 is a scenario for Aleph One - the port of Marathon 1 to Aleph One. What you want is a map editor for Aleph One. You could make maps for use with the M1A1 shapes/sounds/etc, or for other Aleph scenarios.
The current standard for this is Forge, which can be gotten from the Trilogy Release Page. But Forge is very old and outdated, and only runs in OS9.
There is a new program still in development called Pfhorge, which can be gotten somewhere... but it's not finished yet, and doesn't work quite the same way Forge does anyway.