An easier alternative to repairing the damage is to use one coat of mud and then cover the rough repair with a preformed medallion. A plastic medallion can be installed using caulk to stick it to the surface or using the fixture cover plate to hold it in place. You can also get a plaster medallion, these should be installed using setting joint compound or plaster of Paris to attach it to the ceiling. Keep pressure on the medallion to hold it in place until the mud has set.
Hold the wires together so the ends match up and twist them together in a clockwise direction using lineman pliers. Make 1 or 2 turns to form a solid union being careful not to dig into the copper with the plier jaws.
Use a drywall saw and knife to cut drywall and gypsum board plaster, use a reciprocating or keyhole saw to cut wood lath plaster. With the framing exposed, use a small saw to cut two vertical kerfs in the plate about an inch apart and then use a wood chisel to chip out a notch about ½inch or so deep, between the cuts.
Hold the fixture and splice the wires together, cap them with wire nuts and fold them up into the fixture box. Slide the base plate over the wires and thread the coupler nut onto the threaded rod to push the base plate up and hold it in place. Adjust the position of the base plate by turning the coupler until it’s snug against the ceiling.