Unbrako makes a very high strength screw with an unusual round head. They are readily found at specialist bolt and fixing places, and may be easier to get than such an unusual genuine replacement part. You would need one threaded all the way.
I don't know if one would fit in the hole and seal, I have never damaged mine, it is just a suggestion, but you could take the other half of the fitting along to find out.
I don't know if one would fit in the hole and seal, I have never damaged mine, it is just a suggestion, but you could take the other half of the fitting along to find out.