Fiat keys were quite basic then, get your hands on a selection of old Fiat keys (if your in Italy I`m sure your family, like mine, will have several sets to long gone Fiats! if not, ask your local garage (if you know them) for some sets of 70`s & 80`s Fiat group keys.
Gently slide them in & out the barrel (the last 1/3rd of travel) while softly turning clockwise, jiggling them up & down gently at the same time. Eventually one will turn.
You then remove the barrel by turning the key to the 1st position, then push the pin in, this pin is located the side of the barrel housing (after removing the plastic column trim), once pressed, pull the barrel out the housing using the key to pull it out.
You can then change the lock or get a key cut to fit the barrel at a locksmith.