I really liked the rules for using Stars without number as a fantasy engine. I wouldn't do it I like the rules for Labyrinth lord too much to use SWN as the wholesale rule system for fantasy gaming but I do like the rules given for magic use and strain with are very very cool. See the Mage below.
I've reproduced them so you don't have to back link and because I want to have them to hand. Here is the link for the whole article. http://www.sinenomine-pub.com/?p=143
Mage: Mages are in all ways equivalent to the Magic-User class <<snip>> save in the way in which they prepare and cast their spells. At the beginning of the game, the player and the GM take turns picking eight spells from the combined first level spell lists of clerics and magic-users, each choosing those spells that best fit the Mage’s education and style of magic. These choices form the spell list for the Mage. New spells can only be added through research, or the discovery of some arcana utterly appropriate to the Mage’s style as determined by the GM. As the Mage acquires access to new spell levels, this process is repeated to form the list for that level. Mages can start with the knowledge of any two first level spells on their list. Further knowledge must be acquired from a teacher who knows the spell or a scroll inscribed with the enchantment.
Mages must prepare their chosen spells each morning, performing the necessary rites, meditations, and study in order to fix the spells in their mind. A mage can prepare as many spells of each level as is allowed by the spell progression table for the retroclone being used, plus the Mage’s Intelligence modifier. A first level mage with an Intelligence of 14 could prepare two first level spells, for example, while a 5th level mage could prepare four first, three second, and two third level spells.
Once a spell has been prepared, a mage can cast it at will. A prepared spell is not consumed in the casting. Instead, after each casting, the Mage must make a Mental Effort saving throw at a penalty equal to the spell’s level and a bonus equal to their character level. If the save is failed, the Mage earns one System Strain point and becomes Fatigued. A Mage that is already Fatigued falls unconscious for 1d4 rounds after failing a strain save, awakening Fatigued at the end of it. A Mage with maximized System Strain cannot cast any further spells. A half hour of rest eliminates Fatigue, but Fatigue cannot be banished by magic.
A Mage that prefers not to risk accruing System Strain may omit the save and buffer the enchantment with his own vitality, automatically becoming Fatigued but avoiding the expenditure of a System Strain point. Conversely, a mage may choose to “overcharge” a spell, hurling more energy into it that he can safely channel. For each additional point of System Strain spent in boosting the spell, it does +1 damage per die of effect and range and area of effect is calculated as if the caster’s level was +1 higher. Any number of available System Strain points may be spent on overcharging a spell, but the Mage automatically takes an unavoidable 1d4 damage for each point spent, with a Physical Effect save for half.
Is that cool or what! I'll use this in Here Be Dragons or OSR hose rule set as its close to what we have been using but much better.
Note - to really get this rule I had to go read up on "System Strain" in the SWN rules. I recommend you do likewise to get clarity on the implementation of this rule.