On April 14, 1999, about 1700 eastern daylight time, an experimental Zemp Avid Catalina, N9224T, registered to Microlite Jet Engines, Inc., operating as a 14 CFR Part 91 personal flight, crashed following an in-flight propeller blade(s) separation near Sebring, Florida. The amphibian aircraft was substantially damaged, and the private-rated pilot and a passenger sustained minor injuries. The flight originated from Avon Park Municipal Airport (AVO) about 1 hour before the accident. According to the pilot's statement, the flight was at 1,000 feet agl, in cruise, when he heard a "bang", experienced severe vibration of the whole airframe, and when he reduced power, the engine stopped running. The pilot and his passenger determined their only option was to try a forced landing to a clear area within an old citrus grove. As the touchdown area emerged clearer in the pilot's view, he decided to leave the main landing gear up on the hull configured airplane due to the uneven terrain. Upon landing, the right pontoon collided with a small tree/sand dune, and the airplane ground looped and came to rest inverted. Additionally, according to the pilot, postcrash examination of the airplane by he and the passenger revealed two of the three composite propeller blades of the pusher configured engine were missing except for about 10-inches from the hub, outward. Also missing was the engine's front/left side exhaust pipe. The stub of the exhaust pipe remaining shows a 360-degree circumferential crack. An engineering drawing supplied by the pilot detailing the failure is included in this report under, "Other Pertinent Forms and Reports". According to the FAA inspector, the pilot and his passenger, both Swiss nationals, were in this country for only a short time during the 1999 Sun 'N' Fun EAA Convention at Lakeland, Florida. Before the inspector could personally examine the wreckage, the pilot had removed the engine, sold it at Sun 'N' Fun, stored what was left of the airframe in a "T" hangar of a friend at AVO, and returned to Switzerland. Statements made by the friend are included in this report under, "Other Pertinent Forms and Reports".