20 will work....
if it's too long it'll clip the rudder. Most of the new motors, including the 2 strokes, have a much higher profile cowling than the old Johnson seamaster. Therefore you might have to modify the opening in the transom a bit to accomodate it. I never did that, though. My answer is to remove the cowling when I kick the motor up, and just keep the canvas cover on for protection. I leave it down unless I'm racing (rare) or on a very long sail (also rare). It's been like that for over 5 years, no effect whatsoever, although the climate is pretty mild in San Diego. I have a 9.9 tohatsu 2 stroke, it's a terrific motor, highly recommended. It has a remote box, electric start and alternator. I bought it used from a dealer who took it as a trade in for a 50 HP from one of those Mac 26x things.If you opt for a 4 stroke, realize that they are much heavier than the old Johnsons. You may consider at least reinforcing the motor mount area. Anyway, tohatsu has a new lightweight 4 stroke that is getting rave reviews.