Producing Kefka's god form as an enemy in FF7 wouldn't be too difficult. His stationary pose in Dissidia is just about identical to Safer Sephiroth's pose. Compare:
KefkaSephirothWorking on the Safer Sephiroth model, you could change Sephiroth's hair to Kefka's feather, reposition the wings, change two of them to black wings (Vincent Chaos wings are fine), transform Sephiroth's black wing into an arm, add on the legs and loincloth (they won't be able to move, but it doesn't matter)...
I see it being very doable, except that Kefka's face may need to be made from scratch (you can just take the torso from something else in game, like a Moon Gigas or Ifrit).