Yikes! ~_~  I feel for you!

I assume the new lines are just drawn on the surface of the
cel? The first thing I would do is evaluate how much
*original* paint/linework is on the front of the cel and
possibly try to remove the unfinished fix. Before doing
this, I would try to also evaluate the degree of line
fading - can you still easily see the original linework or
is it so faded it's barely visible?

After that, I would do one of two things:

1) Leave it as is in it's faded condition. Unless the
fading is so severe you can barely see the lines, I
generally don't mind it too much - especially if the fading
is even over the whole of the linework. (Below is my most
faded cel - a cel of Yuki from Master of Mosquiton. I
really don't think the fading detracts at all.)

2) Get a small pile of acetate layers and try to make a
"correction" layer for the lines. If you mess up, move to a
new acetate layer and part "B" of this plan: Keep trying. ^_^;

Lots of luck to you!!

Many Sharp Smiles,
--Drac