This should not be a flush valve issue, as it's action is to replace the expelled water.
Lift the "flapper" and run your finger over the seating surface for the flapper and if you fell any irregularities, then replace this part.
The toilet model number is usually found on the underside of the tank lid.

To clarify,I replaced the entire flush valve assembly including the flapper. I thought perhaps the gasket was old and decaying and a new assembly would be the best option. I removed the old assembly with a special tool that came with the new assembly. I have tightened it sufficiently (I think), but water is still leaking into the bowl. Could water be leaking into the bowl via the fill valve? Thanks for any advice.

Drain the tank, remove the bottom fitting and dry the tank.
After ensuring that is clean, place s bead of POLYSEAMAEAL around the hole and reinstall the fitting, remove any caulking residue with your damp finger and allow to set for an hour before filling with water.