For the Dopietta/Side by Side 20 gauge Classic, the Pedersoli manual lists the powder charges as 70 - 90 grains max. for loading with 1 oz. of lead shot.
There isn't any listing for a round ball load for this particular model. Perhaps that's because some of them come from the factory with threaded choke tubes installed.
However, Pedersoli does make another 20 gauge model that's a flintlock named the Indian Trade Musket. In addition to listing the exact same shot load for it, the manual also lists the recommended powder charges of 54 - 80 grains for a .614 round ball with a .010 patch.
That doesn't mean that your shotgun has the same choke constriction and bore diameter, however it's possible that it does and would work. But determining that would entail at least measuring the diameter of the bore at the muzzle. A smaller round ball and a thicker patch could also work depending on the choke and/or bore dimensions.
A 20 gauge cylinder bore should measure about .615.
Track of the Wolf lists available round ball sizes between .595 - .610.
Commonly available patches usually have a thickness between .010 - .020 which also includes .015 and .018 pillow ticking.
POWDER MEASURE SETTINGS TO
MEASURE OUT OUNCES OF BIRD SHOT
50 grain setting = 3/4 ounce of shot
60 grain setting = 7/8 ounce of shot
70 grain setting = 1 once of shot
80 grain setting = 1 1/8 ounce of shot
90 grain setting = 1 1/4 ounce of shot
100 grain setting = 1 3/8 ounce of shot
110 grain setting = 1 1/2 ounce of shot
120 grain setting = 1 5/8 ounce of shot
Folks often load an equal volume of powder and shot, then adjust either the amount of powder or shot volume up or down depending on the performance and pattern desired.
There are also different loading methods and components that folks use for loading their smoothbores depending on whether they prefer traditional or more modern components.
Here's 2 videos about loading a fowler flintlock, one with round ball and the other about loading it with shot.
TVM Flintlock Fowler part 1 Loading with Round Ball
TVM Flintlock Fowler Part 2 Loading with shot