The problem is I think the aircraft is already de-registered. It hasn't flown for decades. But in order to import it to the US register, I'll need "official papers" certificate of export etc. Can ENAC do this?
No way. You might as well be exporting a grand piano as far as ENAC is concerned. Are you sure that the aircraft was actually de-registered by the previous owner?
AFAIK Italian registrations do not expire by themselves, irrespective of CofA renewal, but if the previous owner did indeed de-register it then it is no longer an aircraft as far as the authorities are concerned. Actually, for an aircraft to be legally de-registerd it has to be reported as sold or otherwise transferred to a foreign register, written off in an accident or voluntarily scrapped. If the previous owner did report it as scrapped it might not be a good idea to let the authorities know that it actually wasn't.
I guess in the US you would have to go the experimental route, in the UK perhaps a permit to fly?
Ciao,
DG800