I like my 4 cyl mule. More horses and it is made by Willys!
If your mule is an A5 you would have to drill holes for the extra muffler.
If it is any other, you dont. You can also lower the deck if its not an A5.
But alas, Chuck is right, if it had a 2 cyl it would be wise to keep it that way.
EX C-141 FE CAFB SC
M-151 A2, M-274, M-274 A5,
M-38, M-561, M109A3,
M-416, and lots of parts!