If you can drill through the broken off shank (careful to not go too far.. or your drill will come out through the bottom of the hose..
you'll be able to get an EasyOut or stud extractor into it and that should unscrew it.
The Cinquecento has a similar arrangement and I've looked for any radiator bleed screw that might do the job.. but they (e.g. 71765572) tend to be M8 so too fat for the hole which seems to be an M6 or M7 (can't remember now).
Anyhow, you can get a regular plastic number plate screw into that hole, if you don't have a bolt with a flanged head .. but you will need to try a different O-ring. It has to fit the shank of the bolt, not the O-ring groove that is normally covered by the wide head of the screw.
If I turn up a better solution I'll post it. Someone once mentioned that the Land Rover radiator screw fits.... and you can buy those... but I dunno if that's true. They could be just the same size as the radiator bleed screw.
Ralf S.