That's what I suspected too - rotting wood underneath the fiberglass stringers. I will get a better look once the engine is pulled. If indeed the wood is rotted, then no choice but to rebuilt the stringers. The replacement motor has the same exact footprint. Its just that if the studs could be removed completely it would simplify engine removal by sliding the motor out on its mounts over the stringer onto the cabin floor using a come along.
Oh well, life is beautiful, but never simple!
Happy sailing to all!