I don't know if there was a set of "plans" per se, as the foamboard construction of the giant Mud Duck doesn't really lend itself to that sort of thing. I have one of the small versions that they sold (the Cricket) and it didn't have plans, just an instruction manual for assembling the parts. The only real way to get a set of plans for either of these planes (unless one was produced that I know nothing of) would be to find an unbuilt kit for it and then trace all of the parts out on butcher paper or the like. I do have one of the unbuilt Crickets (as well as an assembled one) but don't have one of the giant Mud Ducks or Ag Ducks.
As far as the original Mud Duck goes, you can get a set of plans for it if you wish through the AMA plans service. Look in your Model Aviation magazine for more details; the construction article was in the May 1992 issue of Model Aviation and can be found in the archives. I have one of them that I originally flew with an OS-26FS that I now fly with an FA30 Saito. They're great fun & not that hard to build; give one a try!