Hi fella...The problem you have is that you have to be drastic to be kind...If you don't have the stringer standing proud of the collapsed glass then you can syringe resin into pre-drilled holes. Drill a hole,through the skin the size of the syringe tip. A hole is drilled at both ends of the delam area so air / excess resin is forced out as you squeegie resin through the delam area.The deck area is then clamped/weighted down. I under catalyse this process as the heat generated even at a 2% mix is great as it cures under the skin. Razor blade the excess drying resin blob over the drilled holes. 4oz cloth patch / hotcoat over holes to seal.
Obviously, use clear resin if there are spray jobs but you will have to pigment the resin if board is white as you will see a resin outline under the skin if you don't.
The other is drastic in that you have to cut the damaged skin away to expose the foam area.Ensure foam is completely dry as major delams usually have let water in over the years ( look for brown staining).You can either rout out an area to replace new foam in ( mix a little resin/ some Cabo silica to thicken so it fills voids) Coat foam block & deck with resin,press together.Weigh it down & leave to set.Re-shape the foam to original profile then apply cloth ( sand down original skin to 'feather' the new cloth & create a strong bond when glassed. Do a hot coat when dry & finish off with a thin gloss coat if original board had one.
If you don't want to go to the trouble of cutting / shaping, you can just cut the damaged skin away,mix resin/ cabo,fill the sunken area,sand, apply cloth/hotcoat. This means though that your board will be heavier as you have compensated foam for resin. It's your choice.
There is also a method to inject foam which raises the delam,bonds the skin but you need industrial injection equipment for this.
Hope this helps.
BEAN Surfboards/ Repairs.