You can add another layer on top of your existing 
cel, and trace the lines on that layer. This 
method doesn't effect the original cel in any 
way. 

Another way, is to trace the lines on the front 
of that faded cel directly. This way is easier, 
but if you make a mistake, you are pretty much 
screwed. Also this second way may not be suitable 
for reselling, because some collectors do care 
about fixed cels.