The smallest motor you should put on a 20' is the 2-stroke 85 hp Yamaha, which produced an honest 94-95 hp.
I think the best motors of the last 10 years for an original 20' SeaCraft that are readily obtainable for under $4k, because of their power to weight ratio, may be:
- the 2-stroke 130 hp V4 Yamaha at 358 lbs, or
- the '98-'04 2-stroke 130 hp V4 Evinrude at 349 lbs, or
- the '00 to '05 Suzuki 2-stroke DT140 I4 at 364 lbs, or
- the '99-'04 Mercury 125 hp Saltwater I4 at 347 lbs, or
- the '02-'08 Tohatsu/Nissan HPDI 120 or 140 hp I4 at 347 lbs.
All are under 370 lbs, all produce at least 125 hp at the prop-shaft, all have good endurance records when meticulously maintained.
If I had a good service-providing dealer for any, I would choose the Yamaha. However, since the best local service in my area is for Mercury and Suzuki, I would choose on that basis. Never buy a motor for which you cannot get excellent local service. You need a good relationship with your outboard service provider.
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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