Join Date: Jul 2007
Location: New Zealand
The seat foam is available as a replacement part. I believe the part number is 46816166. I could be wrong (and check the price with your dealer) but that's the number I looked up on ePer for a seat base foam padding to suit the Momo leather driver's seat of my 2003 156.
Alternatively an auto upholsterer can carve a replacement from chunks of foam (of varying densities) using an electric knife. Fitting the covering is probably more complex than you'd think (involves metal rods in the seat foam and little triangular rings to squeeze into place, working with limited space) so may be best left to the specialists.
'00 GTV V6, '08 FIAT 500, shell and parts for a '71 FIAT 850 Coupe