Your Muse is drunk. Your new first line is debatable perhaps, but Marsilius's is better. The "on a ," being unstressed, don't disturb the meter and make the line flow better. Your new second line simply doesn't work, unless one comes down hard on the first "A," which isn't the way English is spoken and would also require stress on "young." Or, to make the meter work, one would have to make "comely" three syllables or "'proached" two. All three options are too hideous to contemplate. Miss Titus would roll over in her grave. (She was my third grade teacher, and may she continue to rot in hell, miserable old bitch that she was).