Complex sentence: From serial number 543 on, however, D.520s used the 12Y49 engine that had a slightly higher rated performance than the 12Y45, although the German Armistice Commission explicitly prohibited replacing the original engines with the more powerful 12Y51 or 12Z engines.
Split into simpler sentences: From serial number 543 on, however, D.520s used the 12Y49 engine that had a slightly higher rated altitude than the 12Y45. But the German Armistice Commission explicitly prohibited production of the more powerful 12Y51 or 12Z engines.