Each electing small business trust, as defined in section 1361 (e) (1) of the Internal Revenue Code, is subject to tax at the highest rate under s. 71.06 (1q) or (1r), whichever taxable year is applicable, on its income as computed under section 641 of the Internal Revenue Code, as modified by s. 71.05 (6) to (12), (19) and (20).