dont think new shells are available, tho possible at a price if fiat still hold all the press tooling to make the shell.
Used to have an X1/9 and bertone who built the bodies on them still have all the tooling and would supply new doors at £300 a pop.
In the mk2 the shell on 2wd is different to the 4x4 in that the rear suspension on the 4x4 is a completely different set up having leaf springs on the back as opposed to coils on the 2wd.
Cant think of a body difference on the selecta tho.
All share the same base code of 141A but have diffent codes for transmission and engine type for example, my 4x4 is 141AE53B, A 750 with 4sp Gearbox is 141AA43A and a 750 with a 5sp box is 141AA53A and a 1000 5sp is 141AB53A and so on and so on......zzzzzzzzzzzzz lol