Complex sentence: The V12 distillate engine was from General Motors' Cleveland subsidiary, the Winton Engine Company--not, as is usually reported, the Electro-Motive Corporation, which merely supervised installation of the engine into the Pullman-built body.
Split into simpler sentences: The V12 distillate engine was built by General Motors' Cleveland subsidiary, the Winton Engine Company. The Electro-Motive Corporation (EMC), supervised installation of the Winton engine into the Pullman-built body.