1965 Venetian taxi plans

What kind of documents will Portugal accept as an invoice or proof of ownership?
You could register the boat in the USA, which will give you proof of ownership. Then have it surveyed and certified it complies with EU standards.

Importing a boat into Portugal https://algarvedailynews.com/nautical/181-importing-a-boat-into-portugal

 

You may be interested in this drawing I found in a book about venetian books I bought in Venice where I was on holiday years ago.. The superstructure seems to be nearly the same as your boat has. The year of build was 1954.

 

In case of, have you try to contact the Cantiere Tagliapietra specialised in restoration/refit of wooden Venitian taxis, among others activities, may be they can inform and/or guide you :
Introduction - Venetian Wooden Boats https://www.venetianwoodenboat.com/introduction.html
Contact : Contatti - Venetian Wooden Boats https://www.venetianwoodenboat.com/contatti.html