I am the new owner of a P33-2, also with the original bladder style holding tank. When we were shopping, we encountered a few P33-2s that had upgraded the tank and I was planning on doing exactly what they did...
There is plenty of room in the cockpit locker. They simply removed the bladder, secured the board that covers it, and mounted a new tank on top of the board. By my measurements, it should be easy to fit a 20-25 gallon tank (of the normal, prefab tanks available at west marine/defender/etc.) and still have plenty of room for the various junk I throw in the locker. I am sure you could fit a much larger tank in there.