Is this a plastic screw? Unless it is something really special, if you take a new one to the hardware store, you might be able to find a stainless steel or brass (not plated) screw that would work. Well, maybe take the old valve and see if you can find one that will screw in nicely. On throw-away assemblies, especially low-cost ones, they rarely stock replacement parts as they expect you to replace the whole thing. Also, you could check to see if a standard replacement valve, say from Fluidmaster, would work...those tend to be quite sturdy.