S Corporation Modernization Act of 2020 This bill amends the Internal Revenue Code, with respect to the tax treatment of S corporations, to allow S corporations to increase passive investment income from 25% to 60% without incurring additional taxes; eliminate a provision terminating the S corporation status of corporations with excessive passive income in three consecutive years; allow any S corporation to have individual retirement accounts as shareholders; and allow an adjustment to the basis of an S corporation's assets upon the death of a shareholder, in the form of a 15-year amortization deduction.