Technically, then, if you're using a transliteration from Greek, which is what it would be, it would use a k. See, the word, though Persian in origin (ie. pertaining to the Magoi, as Herodotos called them, a class in the Medo-Persia culture), and though it eventually becomes Magus in Latin, is simply Magos in Greek. Now, Greek actually does a lot of things similar to English (or, maybe the other way around.) For example this 'ic' type ending when referring to something pertaining to the noun spoken of. In that manner a place like, say, Karia, has the term 'Karian' (ie. the adjectival form) spelled 'Karikos.' Essentially, it is then a Greek adjectival ending, and I'd wager the same practise was true in the English (only we use a Latinized 'c', but pronounce it as a k anyway.) I'd think that when we used the word 'magic' originally, we meant 'pertaining to the doings of the magi/magoi' and only later did it become a noun, developed from that adjective root. What this all means is that Alastor Crowly was ridiculous in trying to spell it in some sort of way with a c and a k both. If he wanted to spell it in some sort of 'original' way he'd have had to write it Magik, or else Magikos. Plural would be Magikoi. Shame on him, especially as he already has a Greek name! (Alastor meaning 'unforgetting', personifying an avenging spirit.)