By "retrofit", do you mean a window that fits into the existing frame??? If yes, they do not have mounting flanges. New construction windows, that require complete removal of the old window sashes and frame, have a mounting flange.
You can usually cut off the mounting flange, if the window is the correct size to fit into the old frame.
You can usually cut off the mounting flange, if the window is the correct size to fit into the old frame.