repeated capsizing

Not sure about the Capri... but most of these smaller boats have the deck sealed to the hull in a permanent manner. Some Sunfish have had the deck successfully separated from the hull and been resealed, but its not something that I would recommend trying. My bet is a professional shop would just tell you that it can't be done.

Typical for gaining access to do the modifications you list would be to cut holes and install "inspection ports"
Its pretty easy to get up to 5 inch hole saws and a 5 inch dia (nominal... actual hole you cut may be 5.5 inches) port is under $10. Cut the hole and do the work then install the port since the ring will reduce the hole available to put your hand through. Covers either snap in or 1/4 turn and then lock in place.
Lots of threads are in the Sunfish and Laser sections about installing the ports and doing work through the holes in the deck.

Ports are also available in other sizes including large enough to crawl through.