[Dukedesmo]

Rubber seal should be replaceable though not sure if available or if you might need to make a new one?

I dismantled filler cap assembly as I had ignition/fuel cap/seat lock all with different keys but managed to swap/fiddle with the innards so that they all work on the one ignition key and, though it was some years back I'm sure the rubber is replaceable.

If you do need to make one, ensure you get the correct rubber, the old seal on my Guzzi tank was worn out/leaking so I made a new one from a sheet of so-called 'fuel resistant' material but the first time I stopped for petrol it had turned to snot from the fumes.

Bought some Viton (IIRC) and it's been fine for the last few years.