You should say "the right modern greek pronunciation" is Ma-yeé-a, in ancient greek (whose pronunciation is presumably most world-wide known...) the pronunciation is assuredly ma-geï-a.
The greek pronouncation of the word Mageia (Μαγεία) is Ma-yeé-a . The yeé is the same when you say in english 'young' the first syllabe.
You cannot say ma - geï (gay)-a because in greek the ei=i (like ει-ρή-νη=I-re-ne http://en.wiktionary.org/wiki/Irene )Only when it has the two points over the i (εϊ) is ay (like fame)--
Dimitrios Glentadakis