If you're not up to separating the tank from the bowl to repair a damaged flush valve, or if the toilet has it's flush valve and overflow cast into the china, then a retrofit valve-seat replacement kit is a good choice. (Eljer and Crane are two manufactures that may have toilets with these parts cast into the body of the tank.) The kit is an assembly that consists of a stainless-steel or plastic valve seat, an epoxy-putty ring, and a flapper. The super-tough epoxy putty ring adheres the entire unit onto the old seat. The kit can be used over brass, china, or plastic flush valves.
To install a retrofit valve-seat kit, start by shutting off the water and flushing the toilet to drain the tank. Sponge the remaining water from the tank. Remove the old flapper. If your toilet has a tank ball, remove the ball, the lift wire, and the lift-wire guide. Then sand the old seat to remove mineral deposits and to abrade the surface for better adhesion. (You may want to use vinegar if the mineral deposits are stubborn.) Wipe the seat clean, and allow the seat rim to dry.
With the old seat ready, peel one waxed-paper protect from the epoxy (photo at bottom left), and press the assembly firmly onto the old valve seat(photo at bottom right), and turn the water back on.
Epoxy kits are not your only valve-seat repair option. A simpler kit consists of a tube of silicone adhesive, which cures under water, and a plastic replacement valve seat. Sand the seat, and then apply a bead of silicone to the seat rim. Press the new seat in place; install a new flapper; and turn the water back on.