The original 122 and 123 seats were fiberglass with a vacuum formed vinyl cover. Finding good examples is hard and pricey. Finding pieces 2,3,4, and 5 used and then getting a modern seat might be your best route.
The 122 and 123 seat flips. The 102 does not. Those 4 bolts are the seat adjustment. It looks like you are all the way back.
See page 8 of the owners manual here:
The 1250 is actually 3/8”-18 straight thread. It uses a gasket that I consider to be a PITA. Use a 3/8” pipe plug with a little sealer on it. Don’t go go all Hercules on it. The pan is aluminum. Just snug it up good with some sealer.
I use this one.