The rot was primarily on the extreme left and right ends, and the middle of the plywood deck was still in good shape. Using a jigsaw, I made cuts from the front edge to as near to the back wall as I could safely get. Then I chiseled away the rotten wood, leaving the top and bottom layers of fiberglass.
I then cut away an inch-wide strip from the front of the plywood deck -- I wanted some solid wood in its place for the mounting screws to grab.
Next, I cut pieces of new wood to fit exactly in the end sections, and also the new facing strip. These I epoxied into place, screwing and clamping wherever I could. Here is what it looked like once the clamps were removed:
As I write this, I can't find any photos of the finished deck, so I'll have to add some later. The original deck had been covered with Ensolite, but I didn't have any decent scraps of it to use. So I used the same Formica that I used to re-surface the table. It looks good.