Complex sentence: The original model was flown using a Model A Ford engine (which is still the engine favored amongst Pietenpol "purists"), however in the 1960's BH Pietenpol began to favor converted engines from the Chevy Corvair automobiles.
Split into simpler sentences: The original model was flown using a Model A Ford engine (which is still the engine favored amongst Pietenpol "purists"). In the 1960's BH Pietenpol began to favor converted engines from the Chevy Corvair automobiles.