With the boat read to go, I turned my focus to the new core and existing skin. Using the skin as a template it was easy to cut the new core with a box cutter (see in image above). After cutting the core I took it over to the boat for a test fit. The core required a bit of trimming also acomplished with a box cutter.

Core ready, the skin was next. In the image above it is easy to see the nipples created by the former owner's attempt to repair the issue. I used a grinder to remove these then switched to a orbital sander w/ 60 grit paper to sand down the entire skin.

Both the core and skin went to the boat for a test fit. Like a glove! Fortunately, there was enough depth lacking in the new pieces to add a layer of new fiberglass matt to the existing skin.

A note to anyone facing this problem on their beloved craft...
From what I observed of the attempt to repair the soft deck by drilling a matrix of holes and then pouring in epoxy, this method is not effective! The epoxy spreads little. The area may feel more rigid because the nipples of epxoy poured in the holes act like columns to support the deck above. This solution does nothing to stop the progressing damage.