If your deck to hull joint is not leaking, with especial attention to the joint in the keel well between hull and cockpit deck, I'd leave it alone. Take a garden hose and attack it with water from all angles to ascertain the integrity of the joints.
Most of the interior of the Cal 21 is easily accessible for any retrofit job, the exception being the aft ends of the quarterberth areas. A very nice and extremely useful remedy to that issue, and one worth your time, is the addition of a hatch directly under your tiller between the two cockpit seats. You'll find that a hatch of about one foot to the side offers great access to the sternsheets area as well as opens it up for stowage of a great deal of gear. I keep my battery and a small gas tank there.
You'll need to reinforce the cockpit benches adjacent to the opening cut out, ideally with some 1/4" marine plywood laminated in with fiberglas to their underside. Between the access from the quarterberths and your new hatch there is very little of the interior you'll not be able to directly reach.
I just took a photo of my hatch today for someone else and as soon as I get the gear back from my wife, and I figure out the intricacies of Flickr or some such thing, I'll post them here.
From working on mine, while on the trailer, I'd advise purchasing an extral length of shop-vac hose so you can have your shop-vac nozzle close to hand as you proceed. I ran my elec. cord up into the boat and then back down to the vac on the ground so I could plug it in as needed. It goes much easier if you can vac out the mess as you go I found.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.