Your best bet might actually to be to download the version of minecraft (the jar) for the version that mod is for, install the mod as if it was a jar mod, then decompile that. That worked previously (I did it once for Mystcraft, so I could look at the internals and figure out its API by example), not sure how well it works now (had trouble in 1.6.2?).
Give it a shot, at least. Might not leave it in a re-compilable state without fixing errors, but if it works at all, it will work quite well.