Two thoughts and some comments on this.

First up, watching the Firearms FS/T forum over at https://www.muzzleloadingforum.com/ will turn up a double ten gauge occasionally (and this is the right time of the year for it). Pedersoli made a nice SxS 10 gauge you could cut it with no concerns. If you're willing to go with a 12 gauge, there's one for sale now. An original in shooting condition might take longer, but you might find one.

Second, Feltwad (not sure if he's a member here) is into old shotguns. He's in England, and seems to have good connections. Over the years, he's posted photos of some of his collection, and some of his resto projects, and definitiely seems to know what he's about. He may be able to help you find an original in the >10 gauge range--you'd have to ask him. He's on the muzzleloading forum (above), not sure if he's here on NE.

I shoot an original SxS 11 gauge. Nothing special, just an old Belgian gun, probably a market hunter's gun. I've not cut it down yet, but have often contemplated doing so as I use it for ball almost exclusively. I still need to get decent sights on it (currently has a bead in front and a mark on the tang in back), and a ramrod under the barrel.

I've re-worked the crowns on the barrels so that at 50 yards, balls from both barrels with stay in a 6" circle if I do my part. Not optimum, not "kosher" per best gun practices, but good enough for it is for, and more than enough power for anything I'll likely hunt with it.

If you're committed to going with an ML double, gather caps and powder now. They've been in short supply recently.