.32 Crockett
I do not own a BP shotgun, but I do hunt small game a wee bit with a .32 CVA Squirrel Rifle. As mentioned, the .32 Crockett is a currently produced option. Since the OP is familiar with the handling and use (loading) of a rifle, my thought is to stay with the rifle and not go the shotgun route. The Pedersolli double shotgun is a fine rig, but pricey. The Traditions Crockett can be found as a kit for about $400.....recently saw them advertised somewhere in kit form at $369.
The .32 is definitly a SMALL game rifle. The tiny roundball is ideally suited for squirrel and rabbits, groundhogs, coons and so on. A powder charge of 20 gr or so will give many shots to the pound, and plenty of punch for little critters. If you can find a load, the .32 maxiball gives the .32 more punch, but accuracy may be so-so. I honestly think a .36 roundball might be a better GP pot gun, but try and find one these days that is not custom (and expensive). I'm working with the 105 gr Maxiball in my .32 CVA to give it a bit more punch, and to ease the loading process from patch and ball. Accuracy with limited shooting is OK, but my roundball shooting was not that hot either. The old T/C Seneca .32 was rifled 1-20" and intended for the Maxi, but those great little rifles are rare, and again, pricey.