All monsters from OSR bestiaries should work as-is in Knave with no major conversion needed. Here are some guidelines.
Hit Dice/Hit Points: All monster hit dice can be assumed to be d8s unless otherwise specified. To get the monster’s hit points, just multiply the number of hit dice they have by 4 (or 5 if you’re feeling mean.)
Armor: Monster AC (if ascending) is identical to Armor defense. If the AC is descending, subtract it from 19 (if it is from OD&D or B/X D&D) or from 20 (if it is from AD&D) to find its ascending equivalent.
Attack Bonus: Any attack bonus given is unchanged, and can be added to both melee and ranged attacks. If an attack bonus is not given, assume that it is the same as the monster’s number of hit dice.
Damage: Damage remains the same.
Morale: Morale rating remains the same.
Saves: Since OSR monsters usually don’t come with ability scores, assume that monsters have ability bonuses equal to their level, with the corresponding ability defenses.
Example: A typical 4 HD monster would have a bonus of +4 and a defense of 14 in all of its abilities by default, unless modified by the referee.
Designer's Note: Due to the unified 1-to-10 scale of Knave, monsters and NPCs essentially add their hit dice or level to any attacks or saves they make. Obviously this should be adjusted by the referee when it doesn’t make sense.