Disclaimer: I'm not trying to sound like a know-it-all, but this is my opinion based on the phonology of the Maori language.
Well, if you want to go with the official Pronunciation, you could go with /i/, the hard "ee" sound. So Lewa would be "Lee-wa" and Onepu would be "Own-ee-pu".
But I disagree. The letter "I" already stands for the sound /i/ (Nui Jaga, Hali, etc).
I'd go for a more Maori pronunciation of the "e" as the sound /e/, or soft "eh" or "ay" since English does not have the [e] sound, leaving those to be the only possible solutions. So, consistent with the other E's in Tohunga names (Whenua, Epena, ect) that is my answer.
Lewa - Lay-wa
Onepu - O-nay-pu
Kopeke - Ko-peck-ay
Here's the Toa Nuva promo CD where they say Lay-wa Nuva.
You get the picture, essentially, I'm committing "Bionicle blasphemy" because I think e should be pronounced /e/.
Also, Lewa means air in Hawaiian.https://en.wiktionary.org/wiki/lewa