I read this swap on some website (?? the brain is going, can't find it now!!).
I want to do it also as I just acquired a 1604, mine has a 12 Kohler.
My interest in doing this is so I can get that 90 degree gear box and try to make a rear PTO out of it.
What I had read, it is just a "wrench required" swap, no machining necessary.
