3" 1.25 Ounce HeviShot Duck #7.5's would probably give you a "solid" 30 yard+ pattern through that Modified, if you could find some. They discontinued the #7.5's, but they are still around in small quantities.
Or maybe try the Federal HW FCW 1.25 Ounce #7's.
You need premium ammo to help make up for the lack of choke constriction, if you want any capability past about 25 yards with a 20 gauge Modified.
Now at 20 yards, it won't matter what shell you use. You'll be golden at 20 yards.