Thanks for the great information and advice!The hardened lead buckshot will work but you should probably use a cloth patch that is a little thinner than the .018 thick ones a lot of us use with pure lead balls.
If I was shooting the gun I would try some .015" thick patches with those balls. Maybe even some .012 thick patches.
One of the nice things about shooting patched balls is, the ball never touches the bore of the gun. One of our forum members even successfully shot some solid brass balls out of his rifles quite accuretly. As I recall, he also killed a deer using the brass balls. (He was an avid hunter. He didn't live in California but he read about the lead ban in that State so he wanted to be prepared in case his State adapted a similar anti-lead ban.)
A couple of muzzleloaders who write articles for Muzzle Blast magazine (The Bevel Brothers) also tried shooting lead balls that were cast out of wheel weights. As you may know, most of the lead wheel weights are made from hardened lead.
What they found is that the patch thickness needed to be reduced but once that was done, the wheel weight lead balls shot almost as well as the pure lead balls they use to often win shooting matches.