Complex sentence: The role of the elder Elf was written with James Cagney in mind, however Cagney at this point was very weak to old age and though he liked the idea of the movie had no choice but to turn the offer down, so Fred Astaire was considered.
Split into simpler sentences: The role of the Ancient Elf was written with James Cagney in mind; however, with Cagney being very weak in his old age, the elgendary actor could not adequately assume the role, despite the fact that he liked the film's overall idea. Nonetheless, Cagney had no other choice but to turn the offer down, so Fred Astaire was considered.