N1969J is a 1974 Cessna 421B modified by a Riley conversion kit. The aircraft was descending through about 1900 ft MSL when, without previous warning, it yawed left. The pilot reported that the aircraft then immediately yawed severely to the right and began to produce a high-pitched whining noise. The right engine then exploded. Engine debris penetrated the cabin wall and windows and struck the pilot causing minor injuries. The pilot secured the right engine, feathered the propeller and landed at El Paso without further incident. Examination of the engine revealed that the power turbine wheel had disintegrated inflight. Internal examination revealed a broken N2 accessory idler gear which exhibited a fatigue fracture. A loose nut securing the N2 accessory idler gear and N2 speed disc allowed relative movement and wear of these parts. Fatigue of the idler gear caused a loss of drive to the overspeed governor and allowed the engine to overspeed. The manufacturer is redesigning the assembly.