Do not glue the chip back on with anything. o_0;; 
Either leave the cel as it is, or put a touch of 
paint on the area to cover it up.

As for what to use, for tiny touch-up jobs, I 
just use my Liquitex arcrylic paints. If you 
really want to go with the real thing (which is 
expensive) I think they're called. . . (*sighing, 
trying to remember*) Cel Vinyl Paints?  It's been 
covered on the Fourm before - try using the 
search engine and look for "paint".

Keep in mind that anything you do to the cel as 
far as touching it up has the potential to 
devalue it, even if it "looks better". (Example, 
I would much rather have a cel with it's original
faded lines, than a cel on which someone has
"reinked" the lines. Reinked cels aren't as 
valuable, IMHO.) Also, if you go to sell the cel,
you'll want to make sure you disclose it as
"restored" or you might get a ticked-off buyer. 

If you don't know what you're doing, I'd advise 
buying a "junk cel" to learn on, or simply 
leaving it alone.

Many Sharp Smiles,
--Drac