The lock is original and was never a flintlock. Goulcher made thousands of these locks for export and they were used by many many gunsmiths in the 1800's. There are no extra holes in the lockplate where the spring for the frizzen would have been attached. Also the priming pan for a flintlock would have covered some of the engraving.
The drum looks period correct.
Is there a name on the barrel. Without a name there is no way of knowing who made it. The rifle is in the style of Ohio or Indiana gunsmiths.
Is the barrel muzzle coned. This allows a patched ball to be started easily. I would expect the rifle to be 40 cal or less so the muzzle may be coned. The sight may be a replacement also. They look to high. The front sight is usually a "barley corn" style with a tiny silver blade.
I believe the rifle has been cleaned. The patina is not "right". If you run your fingers across the curls on the stock you should feel a ripple effect. Is the butt plate proud of the wood? Wood shrinks with age.
With out handling the gun I can't tell for sure. If you handed the rifle to me at the national muzzle loading shoot in Friendship this is what I would have told you.
It is a fine looking piece. Unbreech it and slug the bore to find the caliber. If the bore is good shoot it and enjoy.