I'd probably just freeform it. Given how many types of magic are present in the series (where even the language the spell is incanted in has a huge impact on the outcome), it would probably be easier.
For the most part it seems to boil down to three branches really.
You have Magic(drawing external energy), Ki(using internal energy) and innate powers(stuff like alternate forms, etc).
For magic itself, you have elemental variances, and each element has its own rider effects.
You also have regular spells(which as far as I know, can be done in pretty much any non-contemporaneous language and resolve fairly quickly) up to High Ancient spells(which take significant time to incant, and are mainly in ancient greek).
You have the eastern style, which uses shikigami and ofuda rather than the incanted methods of western style, but for which high magic is still an incanted ritual. Beyond this is personal magic quirks and sub specialties like potions(stored spell effects), delayed magic(short duration contingent spells), transformations(buffs basically), summons, etc. There is also Rakan. Hes just...Rakan.
Most of this fits into a wide array of systems, since it works off standard JRPG tropes(ranging from MP to magic archetypes), so anything which can emulate those can emulate it.
D&D matches to some degree, and can reproduce most of the effects, but has issues ranging from minor(spells use an point system apparently, effects tend to have large spectacular primary/secondary explosions, spells known system, blasting works, mages only use a bare handful of closely themed spells instead of a wide array of disparate spells, most of these can be worked off modified psionics) to great(casting time is the major one, default incantation time exceeds common D&D rounds, and anyone trying High Ancient cast times in D&D will never finish them).
FATE, of course, always works, but it seems the spells used are not THAT flexible on a per-character basis that you need to use open ended magic like this. Ditto for Mage.
M&M actually loses out somewhat, since it models the resources poorly. Besides practically everything is a Blast-linked-<foo>.
Exalted's sorcery system fits easily(innately MP , inherently long cast time, greater spells take significantly longer, and cast times for first circle spells can be optimized to take less actions), you would probably need to steal the elemental bolt charms for more basic beamspam as well. Drawback is you're going to have to write most of the spell effects. On the bright side Rakan is basically an out of the box Solar.
Ki stuff varies a good bit, theres 'mundane' martial arts like Ku Fei uses(arguably being able to punch a mountain into pebbles is not all that mundane), ki projection(like that 3d jujutsu guy) internally directed enhancements like ninjutsu, close combat blasting like shinmeryu and Rakan. This also seems to work on an MP system, but it does appear to blow less energy away than sorcery. Ki users don't seem get tired as much.
D&D would be throwing in a mix of regular combat stuff with custom ToB disciplines I guess?
FATE, magic by any other name still works the same.
M&M, also similar to the above, but fatigue is less of a concern, so it does model ki abilities better than it does magic.
Exalted would just use Terrestrial/Supernatural Martial Arts, hadn't seen anything like Sidereal Martial Arts yet.
