Unless the exterior panels that were replaced don't match the finish of the originals, I'd leave them in place and just replace the rivets (such as incorrect ones in the picture).
Remount the ribs to the exterior panels. My suggestion would be to drill a top and bottom rivet hole from the inside to set the position of rib, then drill the rest in line so they look correct from the outside.
Good excuse to start buying clecos and cleco pliers
Once all the structure is back in place start the leak testing and get ready to reseal.