The use of the term "Frankenstein" bothers me. It looks to me like you don't have a clear idea of how the bike was conceived to be a bicycle in the first place. For example, what are the options for how braking was to be done? Do those systems correspond to the braze-ons and drillings you can see on the bike? Can you explain all of the features on the frame and what they are for? Until you can you can't see ways to use the frame to make a credible modern bike. Is there any on-line literature to consult? Could it have been for rambling or randonneuring?
Fiorelli was not a random bike house. For decades they made very capable bicycles all the way from tiny childrens' two-wheelers up to city bikes, commuters, sport bikes, racing style bikes, and high-level racing bikes used in the great races of Europe.
The frame looks odd to us, but it was designed for some purpose.
Not that you'll be able to find the "forgotten Fiorelli warehouse full of tandem parts," because if such a thing does not exist for Cinelli, it ain't gonna be found for Fiorelli. But Italian makers generally followed standard interfaces for their parts. For example, if the BB shell widths are 70 mm, most likely the thread fit the Italian standard. Therefore if true you can install any BB that satisfies that fit and threading standard.
What saddles did they use? I don't know, but (my guess is this frame is late '50s) it could have been something like a Schwinn, or like a sprung and wide Brooks, or ... something creative and unique made in Italy. Or it could have imitated ancient styles popular in the 1920s, but still in the parts bins in the '50s. But you need pictures and build lists. Or just perform frame measurements, find out what standards need to be satisfied, and choose something for your friends to use and wow over.
I would try to at least figure out what all the little features and bobs are for before going very far.