Can I repair a plaster ceiling with drywall?
Depending on the thickness of the existing plaster, a drywall patch with suitable support behind it (with "mud" or patching compound over it to merge the edges with existing plaster and achieve a smooth surface) may be a perfectly reasonable solution. Getting it really smooth and level so the patch doesn't "telegraph" through the wallpaper will take some work and patience; sand, mud, sand, mud, repeat until a raking light shows no irregularities from any direction.