RE: crop duster r/c
About a year ago I started looking for a Duster and ran into the same problem. If you want a larger plane you will have to build it from kit form. The kits are not readily available in the States and will have to be ordered. I can not remember the link to the site, but a search should result in some luck. Also I am not sure if it was in the "scale forum" of "giant forum".
I bought a Piper Pawnee from the classifieds shortly after I started looking. It has a 90" wing and came with a US41. It is a Whyte Wings kit and required some modifications to be airworthy, in my opinion. It was design to be a tow plane for their (Whyte Wings) gliders so the wing spar is not the best. I am making it into a functional duster and tow plane combo so I can do both with it. It is in the process of becoming a more scale looking plane than it originally was.
So far I have done a new canopy, fabric wing covering, and some body work. Some of the other mods in the works are wing struts, landing gear, and gravity box. The gravity box should be done this weekend, it will be removable from the bottom of the plane.
If you watch the classifieds you will see one go up for sale occasionally, but not to often. So good luck with your search.
If you wanted a 40 size plane look for the Kyosho Cessna Agwagon ARF it is a real nice plane.
Tim