Carefully drill the heads off of the old rivets and then use a punch to drive the remaining shanks into the hull. Then carefully pry the old mounting rings off the decks and clean up as much of the old silicone as you can using a utility blade.
Once the old ring is off and the deck cleaned up, you can mount the new rings using size 10-24 or 8-32 stainless steel pan head machine screws, about 1” long, with flat washers, lock washers, and hex nuts to secure the new access port mounting ring to the deck. These types of screws should be available at Home Depot.
Before installing the ring, place a healthy bead of silicone caulk around the ring and in the mounting holes. Then install the screws and hardware and tighten them moderately. Finish by wiping away the excess silicone that squeezes out.
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