[Eliot Ness]

Wax paper is a good material to keep glue off the grille cloth. Material for backing board replacements or repairs can often be found at a craft store like Michaels in the form of matting material for large prints.

If the existing backing board is stiff and in good shape those cross pieces are probably not needed. There is certainly no harm in repairing them, but there are a lot of consoles with larger speakers that did not use cross braces. Depending on how tight the speaker presses up to the backing board it will also be adding some rigidity. If the cloth looks taut now with the braces creased and broken, you could probably get by w/o them. Just depends on what you want to do, I'd probably repair them just to keep things original.