I had exactly the same issue a few months ago, only I actually lost the pivot pin. Upon contacting Alfa I was informed that the hook, spring and pivot pin are now obsolete and no longer supplied. Fortunately I obtained a pair of hooks/springs/pins from a breakers (thank god for ebay!!!).
It appears that the pins are a simply an interference fit although mine now have a dab of loctite and so far so good
According to Alfa I was not the first person enquiring about new pivot pins and having looked on the inside of the hood cover I noticed the hook appeared to have been rubbing/vibrating against the cover whilst the roof was stowed away......but on on the side that caused the problem. Maybe it is a common problem!
I now have 2 spare hooks and springs and 1 spare pin......considering getting a couple of pins made to sample!!!