My thought would be to leave it as a 12 bore, but shorten the tubes to behind the chokes, or install rifled chokes if it
has screw-in chokes. It has 2 3/4" chambers as well.
Make a nice SxS 12 bore for round ball shooting with black or smokeless powder - or you could rifle the chokes if fixed
using another barrel as a guide. That would be fun! Slow twists for round ball, patched in cloth, of course.
A ML barrel with 48" to even 80" rate of twist would work just fine. The slowest I have right now, is a 66" twist, fastest
is 48" - in ML's.
A V or U notched rear sight is easily installed in the solid rib by cutting a 3/8" dovetail.