If you have access around the chain plates from below it can be a fairly simple job to cut out he lower skin and re-core it. I have done it many times in fact am about to head to the club right now and cut out a similar spot.
If you have leaking chain plates take a good look at the bulkheads around the plates, they tend to get wet and rot over time. Again they can be replaced but at a cost. Expect issues with most 30 year old boats, they really were not meant to last this long.