paddy, leaking hatches
and portlights are best fixed by replacing the gaskets. If you don't want to get adventurous, Beneteau can probably sell you replacements. Use Armorall or a similar product on the replacments and they will last much longer. Ignore them, and they'll fail similarly. Simple cleaning is not likely to help.
If the "wallpaper" was foam backed, foam often breaks down and crumbles over time and the only solutions are to clean (scrape/scrub with a brush, wire wheel, abrasive block) down to solid fiberglass, then put up a new material or paint
it, install battens, etc. as you choose. You can use a permanent "headliner" type spray cement and try to glue the old stuffback up, but that's a kludge and often an ugly one.