If you're speaking of the rear package shelf, it is a card board type material; depending on your level of desired restoration, either replace it with NOS (they show up regularly on eBay - where I sold mine a couple of years' back), or purchase a quality repop, of which there are several manufacturers to choose from. Those things usually get pretty warped, stained and come apart - I don't know of anyone who could restore one. Be sure to get the one with the front padding attached, as originals were.
Great looking Z, by the way - gotta love those hidden gems and the resurrection from Garage Purgatory. Keep us all informed on your progress -