The 1604 was not offered with a sleeve hitch, but it is the same tractor as a 582 Special. If you buy the special rear plate for a 582 Special (same plate as the 1100 used) you can mount a sleeve hitch on your tractor and mount a rear tiller. Any of the IH built tillers. Make sure you buy the right sleeve hitch, as it has to be for a gear drive tractor.
I suggest contacting IHCCW for the parts. I see he has a new website, and doesn't show that he has the rear plate in stock, but IIRC, his old website showed he does. I'd call and ask.
http://www.ihccw.com/index.php?route=common/home