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Monsters only need hit points and for you to have an idea of what damage they do or effects they cause when your players fail a roll

If you want a reference or compendium to use it together though, you can use Dungeon World's monsters and rules for monster creation. You'll only have to translate other dices to d6 (you can change d8 to d6+1, for example). Also it might not be hard to use monsters of other old school rpgs, like Old School Essentials