John Deere 316 How hard would it be to replace the Kohler K341 with the Onan engine?

I've got a JD 316 with a tired Kohler K341 16 hp. I would like to put the 16 or 18hp Onan engine in it. How hard would this be? I know they made the early 316 with the Kohler single cylinder, a couple years into production they put Onans into them. Im not sure how much different the frame is on the early 316s.