I agree, that looks an aftermarket part which might be hard to source without some more detail, is there anything underneath the base ?
On the plus side, if the rubber O ring has expanded then it's absorbed fuel, swollen and probably softened as well. If you leave it in the open air for a few days, with any luck it will desorb and go back to roughly the original size so you might get away with re-fitting it.
Of course the rubber shouldn't do that, such a reaction points to the wrong material but if it's worked for years without leaks then.... experience trumps theory.
If you want a new O ring though you should be able to measure up where it fits and get something fuel resistant off the shelf ?
Brian