Assuming the gun and the chambers are in good condition, I'd just shoot short 12's in plastic hulls from outfits like RST, Gamebore, Armusa, etc. If you reload, just cut down some plastic Win AA's and have at it. This sounds like one of those situations where it ain't really broke, so don't try to fix it.
I've taken your suggestion this morning and loaded some AA Featherlite Hulls that I've cut down by about 3/8" or so. Loaded with 45 gr. of FFG, Clays Wad (equivalent to the old Win Red Wad), an overshot card and Roll Crimp. Will try it out tomorrow and report back.
I take heart in knowing that this ol gal carries nitro proofs and BP proofs that date to mid 1920's, so I figure I'll be more than safe with these loads.
