They sell those pull behinds pretty reasonably. I'd do that.
I have a 59 on an A. The pulley on the pto speeds it up from 540. Maybe doubles? An air cooled motor runs at 3600 rpm WOT. I'm sure a smarty pants could do the math and tell you what size pulleys to have, but I would just trade to a pull unit. I love the woods on my A - it does a great job. I'm not sure what it would do as a pull unit.