The Genoa cars on our SM were made by Antel.
I assume when you talk about “parts missing underneath”, you are referring to the plastic slide bearings that separate the metal parts of the car from the track.
We source them this past summer from Annapolis Performance Sailing, an Antel dealer. It took a lot of back and forth between Antel USA and Italy before they were sure of the part number, but these were still in stock. They are not, however in the standard catalog.
We received part number ATLG3040A with was a pair of “Antel Car Track Slider Inserts”. Cost less than $20 for a pair. The unusual dimension for these is the length. The cars made for Amel are about 1/3 longer than the standard models. The other dimensions are standard.
Black Point Settlement, Exumas, Bahamas