If you can't find the rivets you can always use a hammer and anvil (most bench vise's have this spot, see pic) to "tighten" the rivets. Lay the rivet on the anvil and hit it with the hammer. That will crush the rivet slightly making the pivot point tighter. Just don't smash the hammer down the first time you hit the rivet.
Tightening up the hinge is more of a temporary fix. The pins more than likely have some wear. The holes are more than likely wallered out. Once the hood is adjusted and you got it right. Open and close it a few time and it's out of alignment and or the back is up.
E body hinges are notorious for worn rivets. The hoods are long and heavy. When I had mine rebuilt the shop that did them told me they make their own oversize rivets.
Some guys think it is an adjustment thing but once you get them repaired and see the difference, you know better.