I think the HGT Abarth has fly by wire TB. If you're using a FIAT ECU, don't worry overmuch about the size of the TB (anything up to 50mm should be fine), but just grab something sensible(ish) off another FIAT with a real throttle.
If going aftermarket, the world is pretty much your lobster as you can forget about the FIAT IAC.
If going aftermarket, the world is pretty much your lobster as you can forget about the FIAT IAC.