Complex sentence: Roles were written and re-written depending on which singer had been engaged, not simply transposing but also adding new material, and some of the new music in later versions may not have been written by Cesti at all.
Split into simpler sentences: Roles were written and re-written depending on which singer had been engaged, not simply transposing but also adding new material. Some of the music in later versions of the score may not have been written by Cesti at all.