Complex sentence: In the end, after some delays, a 420 HP petrol engine was designed, while the main weapons selected was a 75/34 cannon (in lieu of the original 75/18 short gun).
Split into simpler sentences: In the end after some delays, a 420 hp petrol engine was designed. The main weapon selected was a 75/34 cannon (in lieu of the shorter barrelled original 75/18 gun).