I actually did rig 1 (one) 20' with an Evinrude 70 back in the early 80's. It could get the boat on plane but was clearly inadequate. But customer is always right, eh?
I think 85-115 hp is the right HP range for the 20' if you're worried about weight. Your F100 is a good motor . If you're getting airborne in the heavy chop trying running with the motor trimmed all the way down. Adjust your propeller torque tab for that configuration. Alternatively, and better yet, let me suggest adjustable trim tabs. Keep the nose down and it's much harder to get airborne.
The 20' was designed for a single 275-315 lb outboard, or in the case of some of the wide transom cut-out models of '68-73, up to twin 50-65 hp motors at 190 lbs each.
The very first Seafari I rigged back in '83 was a '69 re-rigged from its original twin 190 lb '69 Johnson 55 hp 3-cylinders to a single Mercury 140 ELPT inline 6 weighing in a bit over 300 lbs
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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
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