Just guessing at the age. Can't find a serial number on it. Had old style bailer that I've removed and replaced with newer style, there's no storage area in cockpit and it's heavy as hell even after drying out completely.It's earlier than a '71, so it's seen some use.
I'd press firmly in the damaged area to listen for structural integrity, but in any case, sand down to where you can see undamaged fiberglass. (I suspect this is an area of prior "intervention", but could always be wrong).
Note: It won't be easy. Sunfishes' outer surfaces tend to crumble as they thin. Repair is still possible, but could be daunting.
If the deck doesn't protest your forceful push, you could sand the area smooth, then level it with a layer of Marine-Tex. Still more sanding should bring it back to new. Skill in sanding is important, as paint is unlikely to cover errors.
Marine-Tex is a very hard epoxy, so be patient with the process.