The water tank on the '83-'87 was an integral part of the hull liner. I do not think that there is any "practical" way to R/R the tank. It is fiberglass (nothing wrong with that).
We removed the lid (aft berth) on our H'31 many years ago because we had a small leak. We were never able to re-seal it properly again. We ended up having the lid glassed in place and cutting two inspection ports (one on each side of the baffle) for cleaning.
An alternative is to abandon this tank an use a different area (under the settee, aft lazarette etc).
PS: If it ain't broke, leave it alone. Just purify it and be done with it.