Cels Forum

Depending how bad the lines have faded out.  If the line is
short and you feel like to fix it by yourself, 
you can actually buy some technical art pens to fix
it.  For example, the Itoya Finepoint System is
great to fix this type of ink line, good sizes to use are
.1, .3 and .5   
If you are planning to keep the cel forever, I think
you should fix it since you are going to see it all the 
time.  But if you're planning to re-sell
it later on, I recommend you leave the original way it is.