Track of the Wolf sells balls measuring .410, .424, .429 & .433.
http://www.muzzleloadingforum.com/fusionbb/newpost.php?fid/19/tid/252548/pid/926233/type/reply/
Since the barrel is smooth and not rifled, there aren't any grooves to hold excess patch material after the bore becomes fouled from the first several shots. A true .430 ball with the thinnest .005 patch may fit at first, but maybe not for many shots without swabbing. And then there's the question about whether it's a true .430 ball at all, but rather is a .433 ball or a .429 ball instead.
Smooth bore shooters often opt for a ball that's .015 - .020 smaller than bore diameter for easier loading with a slightly thicker patch. That way the patch has more room to compress when being loaded into the bore.
A .424 ball and a .007 patch might be just about right but that .007 patch will be hard to find. Using it with a .010 patch will require .005 of patch compression which will require experimentation to see if that's too tight or not.
The .410 ball would necessitate using a lubed .015 patch and may fit just about right with only .002 - .004 of patch compression needed. It's hard to say if that's the best choice, but the thicker patch will be easier to compress than a thinner one with a larger ball. Trying to achieve good accuracy with that pistol probably won't be much of an issue no matter which combination of ball and patch is chosen. So based on your measurement I would probably try out either the .410 ball with a lubed .015 patch, or a .429 ball with a .005 patch. If the .015 patch were too loose, then a .0018 patch could be tried with it next. However a .005 patch won't compress much at all and who knows how loose the bore measurement is at the breech end.
If any ball/patch combo. is too loose, then a homemade over powder card or newspaper wad can always be compressed on top of it to hold it in place in the breech and on top of the powder.
Even a loose fitting bare ball can be sandwiched in between 2 wads and fired safely if held in placed on top of the powder.
Good luck and let us know how it loads and shoots once you make the choice.