For some reason I don't have "Strap" in partition 2/menu/ and also I don't know which files to edit in Brawl Box. Can anybody help me?
Yes, you should find it with WiiScruber in the menu "Partition 2-Data" then "Menu", "strap" and there are like 4 archives. Just exctract one depending on your Wii's language configuration.
And, does anybody knows why a modified strap.pac freezes the game when using a Disc instead of the SD card???