It wouldn't have taken much to make him look like he did in ROTJ. I would really prefer his deformation didn't happen. I could believe his power keeping him alive and his body looking like crap because:
1. He was old in ROTS
2. He is very powerful, powerful enough to use the dark side to stay alive
3. Come on it makes perfect sense.
So it could explain why he stayed alive and why he looks like a toad in ROTJ.