I have repaired brittle ones only to have them crack and crumble more. I don't think that its worth the time and trouble. Most of the used ones will be brittle as well if they have been exposed to the sun for 20 years. I looked at buying new ones and I don't like the prices.
I prefer to cut perforated metal mesh to the shape and put rubber trim around it. Hold it in place with three zip ties (Colored, black, or clear).
Mesh (make sure you buy a big enough sheet):
https://www.customcargrills.com/proddetail.asp?prod=Perf_Small
