Yes you can use another plastic grille but you need to determine that is the same TYPE of plastic. Most grills are made of ABS or another styrene-based resin, any of which will work fine. You can also by sticks of ABS plastic used for plastic welding. To determine that a plastic is ABS you can do a simple burn test to help identify it. When you light a piece of plastic on fire it will either stay burning like a candle, or self extinguishes when the flame (Bic lighter, match, etc.) is removed from the plastic. You will also notice variables in the smell, the color of the flame, and the color of the smoke. ABS plastic will easily catch fire and will remain burning when the flame is pulled away. Read more here: Plastic Burn test ID

Another old Moparts thread on the subject Plastic repair