[Fr. Frank]

Okay, X3. If it ain't broke, don't fix it.

Seal every bolt, screw, and their holes that you can find going into the hull, deck, or transom, INCLUDING the ones that have screws in them currently, inside and outside of the hull.
Use a polysulfide/polysulfate sealant like Boat Life, or 4200 by 3M, NOT silicone. If you never, ever want the the thing attached to come loose, you can use 5200 caulk by 3M.
Back the existing screws out carefully, seal the screw and screw hole, and replace carefully.

Keep the motor if it runs well. That hull was originally designed to have a single outboard motor weighing between 220 and 320 lbs, and from 70 to 140 hp.
I sold more new 20' SeaCrafts with 85, 90 and 115 hp motors back in the 80's that any other hp range.
I can tell you that in 1984, the #1 most popular motor we sold on the back of a new 20' SeaCraft Superfisherman was the 297 lb, 115 hp Mercury inline 6 cyl. The #2 in popularity with new owners was the 301 lb, 85 hp V4 Evinrude/Johnson.

If you do choose to repower, at 337 lbs the 90 hp Tohatsu/Nissan 4-stroke is the closest to the originally intended hp & weight for that hull.
The 90 Yamaha is next at 366 lbs.
If you want more power, but still want to stay under 400 lbs, at 375 lbs the 90/115/135 Etec and 90/115/125 Optimax are only 9 lbs heavier that the Yammie and you can get 135 hp for that weight. (The Etecs and Opti's both regularly dyno at 4-7% more than their rated hp, too)

My last 20' SeaCraft had a 90 hp Optimax and performed extremely well, AND GOT 5.7 MILES PER GALLON.