Yeah, I know the cheap replacements are only a couple of hundred dollars, but can it be fixed?
The seat on our Toro ZT has a hard plastic molded base and a soft foam covered by flexible vinyl. Both parts are still sound, but the circumferential bead of adhesive which bonded the two together has failed.
The first thought is to clean off as much of the old adhesive as possible, apply a bead of 3M Weatherstrip adhesive, clamp the two halves together long enough for the glue to set.