Both rebuild kits and replacement valves are available from any of the vendors. Replacement is a little bit more expensive, but it sure is easy. It is a good idea to install an inlet screen like TinCup suggested.
Seems like that screen would be a royal pain to clean if it gets clogged up from a lot of rust fast no?
I can understand how you would think that but in my experience it is not a problem.
I have never had a problem with that with any of the 4 Model A's that I have owned. The screen filter keeps the rust particles floating around inside the gas tank instead of entering the fuel line and clogging the line or the carburetor jets.