I think you could certainly use aluminum tube like the newer SS7 kits. But if you do, it would require filling in part of the square holes in the ribs and probably redoing where the false ribs rest on the tube; sounds like too much work to me. I would just replace the broken area of the wood stringer with a same sized piece of spruce or whatever with good strong scarf joints. This stringer is not of much structural importance. It is only there to stabilize the ribs, and support the aft ends of the false ribs. The aluminum spars and cross braces are where the structural strength lies.