The airplane lost power shortly after takeoff, at 100-200 ft agl. A witness reported hearing the engine backfire several times before it lost power completely. The airplane crashed shortly thereafter, as the pilot was attempting to return to the arpt. Exam of the engine revealed that the ignition harnesses and several spark plugs were in poor condition: several ignition leads were deteriorated; 3/4 of all leads were improperly connected to the magnetos. The spark plugs fired intermittently and broke down under pressure, when operationally tested. Other evidence of poor maintenance was also found, including water contamination in the carb bowl and a deteriorated carb accelerator pump. The airplane underwent a 100 hr inspection 4 months/128 flt hrs before the accident.