Just screw the drywall directly, the old wallpapers and the masonite can also stay underneath so you avoid the demolition work. Renovation drywall (6 mm) is the easiest. If there is a clay plaster layer underneath against the timber, and you tear it down and screw drywall onto it, small pieces can fall off under the drywall in the future, which is why it's fine to leave it as it is today.