Having the existing barrel rifled may be a better option.
; Loading and shooting round balls in the smoothbore might be the best overall effort. We've found 20 bores that hold into 3" at 50 yards for 5 shots using 3 drams of 2F BP and appropriate wadding to fill the case. Use of the gas check base of a trap wad underneath the ball, cup facing upwards, helps centre the ball in the bore and greatly helps with accuracy. A card wad must be used between the plastic gas check and the powder.
; Lyman and Lee or Tanner make many different ball sizes in the 20 bore range, from .570" to .625". You should measure the bore to see it'a actual diameter. If you need tighter than 10" groups at 100 meters or greater range, then by all means, procede with your 'converison'.
; Bill Moody or Ed Rayle in the States will make you any twist or calibre or barrel size you need, for black powder shooting. Chambering and threading for the action will be up to you. Of course, you may have barrel makers locally who can assist you. Good luck.