The rotted part is Item 1 Part number 5070580 - listed at £270 plus VAT. I dont have access to the 312/316 Panda parts catalogue. Maybe someone can take a look. Could be they use the same part?
Edit -
I just found this. - eBay item 293244565381
The later 321/316 4x4 looks to be a modified twist beam axle. It wont help the OP and wont add any value to track day Abarth 500s. Presumably, the front(?) suspension strut and hub are not part of the deal.
Another Edit -
eBay item 353406029303 is for a pair of 4x4 rear swing arms @ £208. The same supplier may be able to help with the centre section Fiat part 5070580.
I was looking at this part number 51857286