Toss the meatballs with 1/4 cup of the marinara sauce.
Arrange an egg roll wrapper in front of you in a diamond shape. Brush the borders with beaten egg. Spoon about 2 tablespoons of the mozzarella into the center of the wrapper, then mound with meatballs (also about 2 to 3 tablespoons) and top with 2 basil leaves. Be sure to leave about 1 inch between the filling and the edges of the wrapper. Fold the end of the wrapper nearest you over the filling. Fold each side over the top, and then carefully roll the wrapper into a tight cylinder, tucking and tightening as you go.
Repeat with the remaining wrappers until all the filling is used up. As you go, remember to keep wrappers you have not yet used, as well as finished egg rolls, covered with a towel to prevent them from drying out.
Add 4 to 5 inches of oil to a heavy-bottomed pot or Dutch oven and heat to 350 degrees F.
Fry the egg rolls in batches, 4 at a time, until the wrappers are golden brown, 6 to 8 minutes. Transfer to a paper-towel-lined baking sheet.
Serve with the remaining marinara sauce for dipping.