Welcome to the forum.
As was said, Dixie might be able to send you a screw for your flask but you will probably have to call them in person.
Another option might be to take the flask to a local hardware or auto supply store and ask them if they have some really tiny metric screws.
You might find one that would screw in.
As for the 4F powder, I'm not sure how you determined it was 4F.
If it really is 4F you don't want to use it in your Walker. 4F powder is only used for priming the pan on flintlock guns. It is too fine and will develop too much pressure if it is used in your Walker.
The best powder for your Walker would be 3Fg black powder.
Some gun stores carry real black powder but you have to ask for it because, by law they must keep it locked up in a special storage area.
If you cannot find real black powder in your area you can use Pyrodex P or Seven7seven. The other synthetic muzzleloading powders will also work in your gun but you should try to get the finer granulation style that is made for pistols.
While your at the gun store you might want to buy some more balls for your gun. The .451, or .454 diameter will work best.
#11 percussion caps should work with your Walker.
