W.C. Scott is a good company. In the world of 150 year old guns, .715 and .719 bores in 13 gauge aren't bad at all, although you certainly want to know what wall thickness is. Also, moderate black powder charges don't produce that much pressure. My experience with original percussion SxS's has been that when you really clean out the bores you start to see the ravages of time. I would certainly get it checked, and the wear around the fences is significant, but I would not assume that it can't be shot.
I shoot a fair amount of skeet with similar guns (my Scott is 10 gauge though), usually using 3/4 ounce of 8.5 shot over 65 to 70 grains FFg. My leads are considerably increased over modern guns, 5 plus feet on the crossers. Upping the powder loads reduces the leads only marginally (as in, I can't tell the difference).
For pheasants I use 1 ounce of 6's over 72 grains FFg, and it works well out to maybe 30 yards. With cylinder chokes nothing seems to work well beyond 30 yards.
