Shelley's Prometheus Unbound - Would be improved if either Asia or Panthea was a ninja -- I'm not sure which would be better. Both wouldn't work, though.This gives them ninja-replacement scores (NRS) of 1, ∞, and 0, respectively. It also works with prose -- Lord of the Rings has an NRS of 0 (the only possible improvement is ninja!Sam, but that wrecks the last line), while Pride and Prejudice has NRS = 5 (arguably).
Byron's Manfred - The whole bleeping cast needs to be replaced by ninjas (except for the spirits, who'd have to be tengus). And then drop-kicked off the Jungfrau.
Keats's Endymion - Ninjas would be superfluous.
Care to rate any others?
ETA: Ninja-replacement in Shakespeare is being discussed here.