Remove one of the hammers from the grid, by applying heat to the base of the hammer to loosen the glue. Bring it to your local music store, and ask them to measure it and order a set of replacement hammers. The hammer is measured with calipers, from one end to the other.
Apply yellow carpenter's glue to the top of the old shank. Then place the new hammer on top of the old shank. Align the hammer so it hits the string evenly, then allow the hammer to sit overnight until the glue dries.
Shape the hammer head, so the felt is flat and hits the strings evenly. You can shape the felt, and cut the edges to make this possible.