If its plastic and you can find another grill for parts or even some other plastic they now have a plastic welder
if its just a pice of the plastic cut a pic of a cheep plastic cd case and glue it in and then paint the grill
typical! I have a 75, and an extra grill. both are a little bit broken. I strengthened the grill supports so to eliminate stress on the plastic when leaning under the hood but I have yet to "solve" the problem. good luck!
I would be checking out AMC and Gremlin forums.