Moesly used wood stringers in the old 21, but I think he was using all glass stringers by the time he built the 19' Bowrider in '64 or '65, and then the 20 cc in about '67 or '68. I know for sure that all the Seafari's had glass stringers starting in '69, and it's the same basic hull as the cc model. The construction/laminate schedule is outlined in this magazine article,
http://www.classicseacraft.com/broch...cle/index.html. The last page contains a sketch that shows how the pre-molded stringers were installed, about half way through the layup. I think Potter may have filled the stringers with foam in his boats, but I believe the Moesly boats like yours have hollow stringers. The only stringer with wood in it is the keelson, and even in it the glass provides most of the stiffness. I believe website founder Trader replaced his keelson with half of a PVC pipe that was glassed over but left hollow for drainage. The I/O's did have wood for the engine beds attached to the stringers, but I think most of the stringers were glass, except for the 21, and even those were covered with glass. Like any beam in bending, most of the strength/stiffness is provided by the outer fibers.
You may find that the boat is full of wet foam between the stringers, so removal/replacement of that would save a lot of weight. Reducing weight in the transom by going to a composite core is a good thing, especially if you intend to install a bracket. In fact, the issue of wood vs. foam in the stringers/transom is a NIT compared to the installation of the bracket in terms of how it will hurt the ride! Been there, done that!
A bracket will shift the boat's CG aft by a fair amount, so you need to be very careful with that. These boats were designed for the largest outboards available in the mid-60's, which weighed 250-300 lbs. Most of the new 4 cyl 4-strokes are in the 400-500 lb range and that's almost too heavy for good balance, even if you just hang it on the transom! Adding a bracket only makes it worse. Use the lightest motor you can find (probably a 2-stroke), and use the smallest possible setback to minimize the CG shift. Consider moving the fuel tank and CC forward to offset the weight of the motor shifted aft. To maintain self-bailing capability, go with a bracket with the largest possible flotation tank (a Hermco) and consider raising the deck a couple of inches. Don Herman said he can build his brackets with different amounts of setback, I believe 18", 24" and 30".
The 20' SeaCraft as originally designed will plane at 12-13 mph, which I think is a very important capability! The boat is light enough to go completely airborne at about 20 kts in seas over 3', so being able to hang on plane at low speed when it gets rough is a big deal to me. My boat would do that when I had a 300 lb motor on the transom, but it took a lot of tweaking to get back there after installing a new and much heavier motor on a bracket! Hope this helps. Denny