I wouldn't leave "Magick" unchanged, as Latin tends to replace ka with ce, when it borrows words and as "-ick" would had been been rendered as -icus, -ica, -icum in Latin.
I however agree that a specific concept might be translated through a specific word, so I would use magicum, -i, n. (as I presume the personal pronoun for Magick being "it" in English) instead of magice, -es, f. or magia, -æ, f.
Magicum ipsius vitæ ars or MAGICVM·IPSIVS·VITÆ·ARS.
[The substantive magicum, -i, n. exists, by the way, and is used as plural: 2 magica , -orum n. PLIN. nat. 1, 24, 99—102 [magic]a de herbis. DOCTR. apost. 2, 2 non [magic]a facies (μαγεύσεις). APVL. apol. 36 de [magic]is meis (sc. commentis).]