Not as much work as you might think: the Star Wars RPG already has a "skills and feats spellcaster class" -- sorta. Adapting the force adept to fantasy and the D&D spells into "force powers" shouldn't be too terribly hard: I've already had one go at it, in fact, although I needed to edit and modify what I did to eliminate a few problems. Using the force causes subdual damage (VP damage) already too, so it's not too different from the high-level explanation of Decipher's system (or descriptions in the novels, either.)
My ground up approach is more conceptual, but involves the fact that magic either has one of two sources: strength of hroa or spirit (why elves and maiar are good at magic, humans rarely are) that allows you to manipulate matter in a way that Galadriel is slightly bemused to call magic, and tapping into the Morgoth corruption that is inherent in the entire earth.
The new mechanics would thus have to have a score (or ability) called spirit, or hroa or something like that, and the higher that is, the better chance you have of pulling off a spell. This score can come from various things (an initial roll based on race at chargen, potential increases as you level up, class-based increases for a loremaster/spellcaster class, etc.) or something like that.
Characters that don't have high hroa scores could still be "sorcerors" like the Mouth of Sauron, Queen Beruthiel, etc., but these are uniformly described by Tolkien as evil. This would use some kind of corruption mechanic, either modelled on WoT Insanity or Star Wars Corruption points.
Anyway, that's just high-level ideas for the "ground up" approach. The new class approach, a rather simple modification of my previous Mage class, which borrows ideas for casting spells from the Star Wars Force-user class, is 95% complete.