A running Ford Model T likely costs absurd amounts of money to maintain because everything about it is likely custom, especially when something breaks.
That's true. I'm doubtful anyone has ever done offset crash testing of a Model T. From what it sounds like, this specific vehicle is driven slow for safety of others and possibly safety of occupants do, and likely not to wear and stress this functional relic as much either.
The risk will be a collision from behind, or a collision from the side at a junction.
In every case I'd rather be in the Fiat 500. The only benefit might be the Model T attracting more attention, but some flashing lights and a garish "Road Maintenance" paint scheme could solve that for the Fiat.