Similar method as the original Montaro game except now the path has changed (https://www.speedrun.com/Montaro/guide/1nj05).

In order to run the All Costumes category you must first delete your same file.

Navigate to your Steam directory, then go to the following path (note this may vary depending on setup and system): /steamapps/common/Montaro RE/Montaro RE_Data/SaveData

Within here you should find a Save.sav file. Delete this file then launch the game and you will have a clean save to be able to run the All Costumes category.