My suggestion would be to put glue down the holes on the bottom of the green plastic seat where they are stripped and let the glue harden. The carefully drill down the center to make new holes the diameter of the original screw shafts (not the threads). The screw the original screws in. The threads should bite and hold fine.
At least that's what I would do. I'm sure someone else has other suggestions. Using a bigger screw is not an option since those screw mounts in the seat would not allow a bigger screw.
Good luck on your repairs!
