Complex sentence: The major changes were; the replacement of the engine with a modified version of the Hispano-Suiza 12Y engine made under licence by the Czechoslovak company Avia, and the replacement of the cannon by a Swiss-made Oerlikon FF 20 mm cannon.
Split into simpler sentences: The major changes were the replacement of the engine with a modified version of the Hispano-Suiza 12Y engine made under licence by the Czechoslovak company Avia. Along with the engine, the cannon was replaced by a Swiss-made Oerlikon FF 20 mm cannon.