Luckily these gaskets can be made at home, if not readily commercially available. They don't have moulded features like some.
Obtain gasoline-proof, high density, flexible material in the correct thickness, and use wad punches to cut out the disc and make the holes.
I dismantled an Orlandi fuel tap of unknown origin that seems identical to those on my Ducati single in most respects.
The (black) rubber disc is 2mm thick. and has 3 x 3.5mm holes, at a PCD of 8mm. The outside diameter is 14mm.
To prevent the disc from moving, the 3 holes in the tap have protruding spigots, as per attached photo.
Your own photo does not show this feature. But it does look suspiciously like it had some hollow dowels that have been removed.
Even if you installed a sealing disc to this tap, you would likely have problems unless something was done to positively locate it.
You could try reinstalling some dowels, maybe made from brass model maker's tube, if you could find the right size.
Alternatively, maybe a 4th hole could be made in the rubber disc, with a small pin installed in the tap body.
fuel tap internal.jpg
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