If I were you, I would directly contact the associations for missing and UID in the countries you think about.
The case is quite old, and it was way before Internet. So, don't rely too much on what you can find online because you'll end up disappointed.
In such case, put yourself in contact with charities for missing and UID persons because they work closely with LE and ME, so they can get info you won't (easily) access through the web, by yourself. For the countries you worked before, if I were you, I would contact the charities in said countries to see what they say. In a few words, you need a fresh start.
Then, you can report the answer here, in this thread.
Charities are more open to independent citizens requests than are LE and ME. So, starting there can be extremely valuable : you'll be surprised with the breadth and depth of info you get from these charities you can't (easily) get otherwise.
It doesn't prevent you from working on Websleuths !! Websleuths and charities complete each other, they are not in a contest.
Other than charities, we are running out of solutions.
For contacting Italian charities and Chi l'ha visto, I take care of it. So, you don't need to work on this task, and you'll get overworked unproductively.