Not sure about usage on an Attalia, but on the slightly smaller Arcadia I own, I can go about 8'ish hrs on my 7 gal tank, with a 16HP Yanmar. If I am not a throttle to the floor type motor, I use less than a gal and hr at 5-6 knots, if I try to go over 6, then it jumps to 1+ gals per hr of use.
As far as fitting a bigger tank. assuming the two have tanks are in equal spots, ie for me under the starboard seat, removing the one and installing a bigger tank would not be that hard, but still an 8-12 hr job if I had to guess on time! I could see a use for a bigger tank if fueling stations were few and far in between, but with a 5 gal cont I keep in that locker, I can go some 12-14 hrs of motoring pretty easy, for me in Puget sound, I'd pass plenty of fuel spots in that time!
She drives me boat,
I drives me dinghy!