That’s not a stock cap so no part number. The oem cap uses an o-ring to seal the cap to the tank.

 

Maybe give Motomillion a call/email? They're the official TMW distributor for the U.S.

 

As above, or failing that take some measurements of your gasket and post here - I'm sure someone will be able to find a match.

 

It’s just a piece of flat rubber, shouldn’t be hard to make your own gasket.