Unable to come up with anything witty to say. I've seen the missing plies of plywood run seven or eight feet up the main frame rails, leaving 1/4" then 1/8" deep furrows missing due to the fiberglass water wicking. I've seen the lacey lower edges of outriggers but less so on my '73 spars.
That sure looks like a good candidate for adding a new spar right behind what remains if the tank etc. clearances allow doubling up.
As pictured there may be a strong argument for LEAVING IT ALONE, no more picking at it, scab in a 90° angle sheet metal stiffener on the bottom edge by tacking it in on the vertical faces near the main frame rails AFTER POR-15'ing the rusty spar one good coat to build up and strengthen what remains.
Once POR soaks into rust & oxides and bonds, the next coat(s) gluing new metal mated up almost* resembles being welded. * I reference the snapped C-channel that lifted with the shell on mine while being POR welded to the tie-down plate. Then the pop-rivets used to hang the belly pan will tie the rest of it together for a long time, just leave clearances for any plumbing etc. running through the lightening holes..
My back tie down area is different than that you've pictured, there is no stamped metal spar at the last cross brace area, it has a 1.25" x 0.75" heavy iron "U" crosspiece to intercept the tie plate. And yes the aluminum shell resembled busted graham crackers once I had it apart.
Replacing the 'entire sheet' is a little too deep for the nice metal above the oxidized strip there, maybe just rivet in a reinforced overlay for the lowest four or six inches. However, I am thinking of omitting the entire back utility hatch and frame versus replacing the tiny sheet strip beneath the hatch as AS provided.
has anti-everything etch/brightener & passivation supplies for aluminum I think you'd ought to consider investing in. ALUMIPREP NO. 33 or METAL PREP NO. 79 then ALODINE 1001. Anything that has aging & weathering on it and all the other susceptible spots get treated...