There are 17 Greek consonants. The only one without a true English counterpart is 'XI.' It's pronounced like the ending in the name 'Bach' and will be represented as a 'c.' Here are the Greek consonants;

(gg, gk, gx, and gc, will be assimilated as ng, nk, nx and nc)
(sb and sm will be assimilated as zb and zm)
The 7 Greek vowels; 
will be represented as: a e ay i o u ow
Diphthongs:
(Proofreaders are welcomed! Initial letters that are aspirated will be noted in the lexicon.)