hmm..imho, buying the gear box is easy, but modifying your wirings for the 4at is gonna cost you...
furthermore, you will most probably face with loss of power due to the gear ratios, do remember that a 1.6 has better power delivery compared to 1.5,
I ventured into this before, ended up disappointed, and money down the drain!
my wira 1.5 auto can only do 380 for 60 bucks, why don't you change back to drop in filter, better fuel efficiency!
just my two cents!