https://wccftech.com/halo-combat-evolved-mod-tools-released-by-343-industries/

As part of Halo: The Master Chief Collection Season 7, 343 Industries also released the official Halo: Combat Evolved mod tools as they had promised to the PC community.

Interestingly, you won't even need to own Halo: The Master Chief Collection to access them as they should appear in your Steam library under Tools. Otherwise, just click this link with Steam open.