Complex sentence: Completed during the Vichy era, a further improved model of the aircraft, designated as the MB.157, featured a heavy and more powerful engine; while demonstrating promising performance, it did not enter production.
Split into simpler sentences: Completed during the Vichy era, a further improved model of the aircraft, designated as the MB.157, featured a heavier and more powerful engine. Though demonstrating promising performance, it did not enter production.