If the bore is in good shape measure it and subtract 10 thousands and you have the ball size.
That type of pistol is usually shot with 35 to 50 gr of powder ,I would recommend black or pyrodex.
With the 10 T gap between the ball and barrel the commercial patching should work or pick up some ticking and rip it into strips, use the short peg on a good short starter to push it a quarter inch into the barrel and cut off with a patch knife or striat razor.
I would also recommend somepatch grease like olive oil or borebutter.
Do remember to scrub the gun with soap and hot water and dry and oil when done shooting.