I think the llater 123 have the foam pad, I replaced mine with a newer used one and used some stuff called batting my wife get from the yardage store to fluf it up.
the seat back is held on with 2 bolts on either side, gently pop off the plastic side pieces.
The seat back is held on with 2 screws at the bottom. remove them and slide the back down.
The arm rest is held in with a large nut.
The seat cover bottom is held on around the lower edge in a folded over piece of metal. push down, compress the seat and pull the fabric down and out of the slot. there is a stiff piece of cardboard sewn along the lower edge and folded up into the slot.
then snip off the hog rings and the cover will come off.
Do it once and you will be an expert

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Charlie