The spell lists from any old-school RPG will work perfectly well in Knave, provided that they go up to 9th level. There are many free lists of classic spells available online.
In Knave, PCs may only cast spells of their level or less, so a level 3 PC could only cast spells of level 0 to 3. Spells are cast out of spell books, which must be held in both hands and read aloud. Each spell book can only be used once per day. Importantly, each spell book only holds a single spell, and each spell book takes up an item slot, so if a PC wants to be able to cast a wide variety of spells, they’ll have to fill most of their inventory with spell books.
Designer's Note: It’s always seemed odd to me that spell levels don’t correspond to PC level in most OSR games. Well, now they do. I also took the abstract notion of spell slots and turned them into something concrete; PCs can cast as many spells as they can physically carry. Boost Constitution if you want your PC to carry around that mobile library.
PCs are unable to create, copy or transcribe spell books. In order to gain new spell books, PCs must adventure for them, by either recovering them from dungeons or looting them from other magicians. The higher the level of the spell book, the rarer and more valuable it is. PCs openly carrying high level spell books are likely to be hounded by bandits and wizards looking to “acquire” them.
When a spell allows for a save, make an opposed Intelligence save against the defender’s relevant ability, usually Dexterity for ranged attack spells, Constitution for life- draining spells, Intelligence for mind-altering spells, or Wisdom for Illusions.
Designer's Note: Note that spell books can be easily re-skinned as rune stones, clay tablets, potions, scrolls, or whatever else fits your campaign. If you wanted a more dangerous, low-magic setting for example, you could make spell books potions or scrolls that are only used once and then lost forever. The random spell generator found in my other game, Maze Rats, can be useful for generating ideas for new spells.