Complex sentence: The Italian Air Force had ordered 258 aircraft most of them two-seat FN.305A fighter trainers and liaison aircraft, a few of the aircraft were completed as single-seat open-cockpit FN.305Cs and enclosed-cockpit FN.305Ds.
Split into simpler sentences: The Italian Air Force had ordered 258 aircraft, most of them two-seat FN.305A fighter trainers and liaison aircraft. A few of the aircraft were completed as single-seat open-cockpit FN.305Cs and enclosed-cockpit FN.305Ds.