Depends. If you have the parts and can fit them together pretty well, glue them back in place. I've used good ol' brain rotting model cement. The other stuff I would recommend is Slo Zap. Slo Zap is a CA cement that sets up slowly and can be used to fill small gaps.
If stuff is pretty messed up and parts are missing, as happened on the Demon, I found a really crappy grille that was undamaged where mine was damaged. The yard operator gave it to me. I grafted the section in. With this technique, I recommend cleaning up the broken area to remove angles and irregular surfaces. Try to have joints between the pieces happen where there are right angle intersections between design elements. Cut the section out of the donor a little on the large side in all directions. File until it fits with a little tension. When you are satisfied with fit, apply the adhesive. I chamfered the edges of the joints on the patch panel and put in a little extra glue. When it dried, I went back with a fine rat tail file to square up the joint. Looks good, even up close. You have to look at the grille from the back to tell that something has been done to it.
