I'm building a similar brand-x skiff and the recommended outboard size is 2 HP or less.

Laszlo

I built the Jimmy Skiff last year, and used a Gamefisher 1.2 hp motor on it a few times.  This very small, very light motor would push the JS about as fast as you would want to go in this boat.  The problem was the noise, smell, and oil the two-stroke motor produces.  Another problem was that this was not a long-shaft motor, and the prop would come to the surface and splash a lot.  I had it mounted on the port side of the transom, and I used my rudder tiller with extension so that I could sit on the center thwart.

I have since bought a 40# thrust Minnkota Riptide electric trolling motor, and this is what I use when there is no wind and I don't feel like rowing, (which is not very often)  This motor will push the JS with 400# of crew and gear at about 4 mph for about 2.5 hours, using a size 27 100 AHR battery.  I mount the 70# battery just behind the mast step, and this added ballast keeps the bow down and smooths out the ride when there is a lot of chop.  This may be a little too far forward, as some motorboat wakes will now come over the bow a little.  I think I will try moving it aft of the forward thwart.  This should be a much better location for proper trim when I am seated just aft of the center thwart, on the side seats I have added.

Are you considering gas or electric?  You may want to double the transom, or add a stiffening rail across the top to keep it from flexing.

Ron - www.jimmyskiff.blogspot.com