I have a '72 Seafari 20', '06 Mercury 90 hp Optimax with 13.25"X21P 4 blade and skeg-mounted Hydro-shield. WOT 33-34 mph, cruise 24 mph at 5.1 mpg. Planes quickly.
Remember, the most common original equipment motors on these 20' SeaCrafts were the 85 hp and 115 hp V4 Evinrude/Johnsons, or the 90 hp or 115 hp inline 6 cylinder Mercury motors. As a SeaCraft dealer, I have even rigged a 20 SeaCraft with a single 70 hp motor, and it did fine. There is also another member with a 20' Seafari rigged with a Mercury 80/85 hp two-stroke 4 cylinder. These hulls don't
need all that extra horsepower.
But if you want to put the extra power on, they'll handle it just fine. Locally, there's a 20' 1971 SeaCraft Superfisherman with a Porta-Bracket and a 250 Yamaha HPDI that runs like a raped ape. (Of course, he only averages 2.5 miles per gallon.)
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury