The engine all depends on how you want to fly. The first one I built many years ago had a Saito 45 and it flew scale like, the next one had a Saito 56 and was more that enough power to do most aerobatic's. IF you are thinking 3d then the 72 should be fine.
After looking oin the web and een had a riend that flies in to Japan gave up on the Agwagon and am building and Air Tractor from Team Duster.

See thread in Kit building.

Jim