On October 12, 2000, about 0800 central daylight time, a Republic RC-3, N6175K, piloted by a commercial pilot, encountered an in-flight loss of engine power and sustained substantial damage on impact with vegetation and terrain during a forced landing in a field near Baraboo, Wisconsin. The ferry flight was operating under 14 CFR Part 91. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot and pilot rated passenger were uninjured. The flight was on initial climbout from Baraboo Wisconsin Dells Airport (DLL), near Baraboo, Wisconsin and was destined for Genesee County Airport, near Batavia, New York. The pilot stated, "Perfect VFR weather. At about 0715 local, the aircraft was fueled and preflighted. One quart engine oil added. Engine was started and run to verify operation. Engine operation was smooth after engine was warmed up. Aircraft was taxied to runway 19. Magneto, carb heat, and propeller operations all checked - normal operation during run-up. Engine response was accurate and smooth. Flap operations checked and set for takeoff. Pre takeoff checklist completed. At about 0800 local, taxied into position. Applied full power and verified normal manifold pressure and propeller RPM. During takeoff roll prop RPM observed steady at bottom of yellow arc, about 2450 RPM. Liftoff occurred at approximately 55 mph. Aircraft accelerated to about 75 MPH and climb rate increased to greater than 250 feet per minute on VSI (observed). Between 250' and 400' AGL airspeed began to decay and climb rate decreased quickly. Pilot reduced pitch attitude to maintain airspeed (no change in engine sound was obvious after power and prop were set for takeoff). Prop, throttle, mixture, and carb heat verified full forward, no change in performance observed. Traffic pattern turn abandoned as climb rate and airspeed continued to decay. Aircraft pitch attitude was continually being reduced to maintain flying speed. Control became difficult (control feel 'mushy') and return to runway seemed impossible. Altitude was decreasing quickly. At this point I estimate height AGL at less than 200'. Aircraft was turned into wind, towards best field. Avoided obstacles and landed in corn field." The pilot stated that the airplane damage was a broken left main gear, damaged right wing sponson, damaged right wing tip, and cracked right wing spar. The pilot listed the mechanical malfunction as, "Intake valve spring failed on #5 cylinder. When spring failed, allowed intake valve to remain open. Failure disrupted entire engine function."