After you have cleaned this thoroughly, measure the barrel. What we particularly need is bore diameter at the bottom (which you will likely have to determine with plugs), breech diameter, bore length and barrel length from muzzle to breech (don't include the cascable, the knob at the back, in that last length.)
Just speculating, the best thing you could do would be have a new barrel made from steel to the old barrel's external dimensions but with a smaller bore that would give you breech walls the same thickness as the bore. That would be safer to fire although you would still need to use a reasonable powder charge.
