I agree, top work the tree with some good varieties. Its not that hard. Cut the main branches and let them resprout. You want to graft in the hottest months when the tree is actively growing. Use cleft graft or side veneer. Wrap (cover whole scion)with parafilm tightly. I like to put a plastic bag over the new graft and tie it to help keep in humidity, like a greenhouse. Take the bag off when the scion sprouts. Leave the parafilm on. Watch videos of Dr. Richard Campbell grafting on youtube. Good Luck!
E.