If I understand correctly, zero ryoko is painted 
on one cel, with a cel on top of that painted 
with just another face.  If you like the face on 
the bottom cel (this is again assuming it is a 
complete cel, not just her body or something)- 
you could just take the top layer off and discard 
it, and remove any remaining paint from the top 
(front) of the bottom cel.  If the bottom cel is 
complete, and painted on the back of the acetate 
as it should be, it will not matter what you do 
to the top as long as you do not scratch it or 
mess up the lines from the copier.