Actually; if this is the cel your talking about 
the line fading doesn't really look that bad. Try 
having a cel where there are no lines left at 
all!!  Toei is famous for line fading (SM and 
Dragonball) My suggestion would be to leave the 
cel alone. Faded cels are apart of collecting. 
Its very difficult to fix them, not to mention 
expensive, and unless the cel was damaged I'd 
leave it alone...