engine hp and transmision ration

[hinemoa]

like most threads i ve read, i have a querry about my new venture

16.5 metre catamaran
aliminium hulls
1.3 at their widest but water line around 900mm
water line around 350mm
weight around 9 ton??
shaft size already in boat is 1 3/4
prop size(pitch unknown)400mm in diameter

the boat was a comercial tour boat, but now gutted from original 120hp cummins diesels(and gbox ratio unknown)
previous speed unknown

basically after a thought of what minimum engine size in each hull i can use as its now a pleasure craft and wouldnt mind if it had a reasonable pase picture of engine bay included
and rear ends