I took a look at Star Wars Saga Edition when it originally came out; looking back, it's interesting that Wizards talked about how it was one of the stepping stones to 4th edition, because it's far more similar to d20 than 4e (imo). They certainly simplified a lot of things down to where they are in 4th edition, but the "powers" mechanism which I would consider 4e's keystone is nowhere to be seen, and the class mechanics are a generalised version of d20's.
Given the options, there are 2 possible starting points for Saga, both with their own advantages and disadvantages.
The 4e data files would be one place to start; they're the simpler choice, and you can edit all the mechanics since you have the source code. At-Will powers can be refitted into Talents, Encounter powers to Force powers, and Daily / Utility powers simply discarded. However, the 4e files don't support classes to the level necessary for complete Saga support - you're fine if you want to take a single class (and maybe one prestige class), but if you want to combine classes you're out of luck. The core tenet of 4e is that you have a single class, a single paragon path and a single epic destiny - multiclassing is entirely based on feats, even with the new 'hybrid class' system they designed.
The d20 files would be the other place. They have the class system already in place, so you can easily create all the different Saga Edition classes and combine them in any ways you want. Talents and Force Powers can be assigned via custom abilities and spells.
If I personally was going about creating Saga Edition data files, I'd probably do it in a copy of the d20 files, with all the standard classes and feats marked with "hide" elements. However, I haven't gone through the book recently enough to identify any "gotchas" that would cause problems. Have you considered that as a solution? If not, it might be worth trying to implement a couple of classes in d20 to see how easy it is before making major changes to the 4e files.
Hope this helps!
