Are you sure you talk about one of the two pins that plunge into the metallic hood cover to lock the back window in place ?????
Because, if so, those pins can be ordered from alfa. They are referenced as available parts and cost a bit more than £10 per unit. (Careful, refs are different depending if you have or not electrical hood).
On the other hand, if it's one of the rivets of the structure that has gone,..., well, there nothing but to replace the complete hood.
FIY, the structure comes with a new hood cover that will need to be adjusted. It's likely that the hood+structure cost £2.5 + £500 will be charged for fitting and adjusting.