Complex sentence: The Fulmar was a two-seat fighter with good range but at a performance disadvantage against single-seater fighters; navalised Supermarine Spitfires were not available because of the greater need of the Royal Air Force.
Split into simpler sentences: The Fulmar was a two-seat fighter with good range, but at a performance disadvantage against single-seater fighters. Navalised Supermarine Spitfires were not available because of the greater need of the Royal Air Force.