Rather than buying a new chair or covering the cushion, you can make it good as new by replacing the seat.
Unscrew the seat you need to replace and screw these new upholstered ones in the frame. An example chair in the last picture, but this posting does NOT include a chair.
These are high end Thomasville cushions.
The front is 19" wide.
The back is 15" wide.
16” from front to back of seat.
Brand new, never used. Most of them are still sealed in plastic.