I recommend you modify 1st edition Runequest, using CHA or WIS in place of POW. Every spell has a point value; you can memorize INT points of spells per day and casting each one expends that many points of temporary POW (which is regenerated at score/24 points per hour). I'm not immediately sure how you'd fit in levels but it's a solid starting point and a very clean, simple system.
Now that I look at it, the changes aren't as extensive as I thought - mostly just reworking the damage for a few spells but I haven't scrutinized the list in detail as the way they present it it's hard to see what has changed. I notice that fireball and lightningbolt are lowered to d4, for example, but we all knew those spells were a little overpowered anyway...