As you have an original fuel tap, I would stay with it.
Do not buy a later one.
Look at the link above, this company, Faak-Tiillmanns specialize in carburetor and o fuel tap parts parts. That is all
They also communicate to you in English.
You will see from their website, there are different thicknesses, of the cork seal to replace.
Which as others have said is a simple job,
I did this about 4-5 years ago, for the similar tap as fitted into my 1951 R25, an original import into the UK in may of that year. and
it seals perfectly
You can also download a very good diagrams, on the various carburetors, by specific model number with all the parts identified down to the tiniest washer.
I can understand dealers saying get and later tap, because the cost of that is enormous, compared to a tiny a cork washer, it is also easier for them of to take a new units off the shelf and post away. The profit on a new tap would of course be much more than on a very cheap washer
But for the sake of originality, buy the replacement cork washer.
The rear of my tap has some numbers and letters inscribed, obviously stamped at the time, but no one has been able to decide what they meant
Perhaps it was fuel flow, all the man who actually did it
The only thing with this company is, they do not take credit cards
I suppose that keeps costs down because of the charge by the card company
Have a good Christmas, and many happy miles in 2019