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
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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.
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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.