Thanks for all the ideas.
Some further info: Knowing how problematical getting a good seal has been, and the large quantity of sealant plus various adhesives now inside, the only practical way of removing the innards would be with a Rotacraft grinder. Everything from the under decks downwards. The sealant has set quite hard. Bound to go through the hull though.
3 mm Lexan is very rigid, much more so than ply. It is the hull that is thin, and maybe slightly twistable. The hull and bulkheads were cleaned as if for painting. The cut edges at least should have permanently bonded.
I like the idea of lining the ballast tanks with fibreglass sheet, but now the only access to the inside of the tanks is through the vent slots. Liquid rubber ? ‘Emergency’ tyre inflator liquid? Air bladders? (if I can get them inside).
My thought of filling the tanks with sealing foam, before filling in the slots, was just to provide permanent buoyancy. However, that would end submersibility.