Need tips on how to restore foxbody intake manifold cover.

The plate? or the entire manifold?

For the plate, sandblast it, paint it black, and then hit the top surface with an orbital sander.

For the manifold, it's painted, so you'll need to strip the old paint down, and prep the surface to repaint it. Given how many people practically give away stock HO manifolds, you may want to hope on the FB groups and see if you can pick up a better condition manifold and just swap them. People pretty much scrap those, so it should be short money, unless someone decides their $20 part is suddenly $200 because someone put up a WTB ad (which does happen unfortunately)