How you repair this board depends upon how badly you want to maintain the originality of the board. And; how much effort you are willing to put into it. You could cut the sunken patch out with a trim router. Mark it with a Sharpie and cut it out. Check the depth and determine how deep your cut out is. If it is shallow you might get away with layering a couple of layers of six ounce in the cutout and then a final layer that would be feathered a couple of inches onto the surrounding deck area. I have filled delams with lite weight spackle or q-cell . Some guys use Pour Foam. You could also glue Divinicell into the cut out. Take into consideration added weight and have a plan on how to hide the repair cosmetically… After repair and sanding, you might be able to paint a deck panel that would cover everything and look original. That’s a decent looking old Bing Bonzer and with the effort if you are willin’. Throwing a deck pad at it is a lazy man’s coverup.