If you want to get a good match to the original plaster, use plaster patching or real plaster on the drywall, greenboard would be bettera s it is stronger than regular drywall drywall. apply plaster in thin coats and try to match the original pattern if present, smoothe and feather to the original plaster level otherwise. plaster will gove a closer match to the original than will MUD and is applied in the same manner as mud buit takes a bit longer to dry. MOH.