If the barrel metal is without defects, the Belgians back then had some of the strictest proofing requirements of the period for muzzleloaders.
If you want to refinish the barrel, use any good rust remover, sand lightly with fine grit paper or cloth, and cold blue or brown to wants. The wood can be stripped of finish with Zip Strip or such. Some dents and dings in the wood can be raised with wet cloth and a hot iron to steam them up. Wood can be sanded lightly and then refinish with one of several stock finishes available.
Word of warning, if it has "NOT" been refinished, any antique value will be greatly diminished if one does so.