I believe that the entire bulkhead is made of plywood, and not a laminate of balsa core with fiberglass on both sides. I would scarf in (at least an 8:1 scarf, and use epoxy glue) a new piece of similar thickness marine ply. It would be preferable if this was sealed with epoxy before you did the repair.
Starboard is almost impossible to glue, so would not be satisfactory for a repair. There is no need for metal--and it might be detrimental, because of corrosion. When you do the repair--"tab" the plywood into the hull bottom interior on both sides, with 4" fiberglass tape if only one size tape.. (several concentric layers would be best--starting with 6", 4" and the 3".) Epoxy and tape work well. If you were using polyester resin (not recommended, because epoxy gives better strength of the secondary bond)--you could use mat, and a final layer of cloth.
It will be best to cut the old rotted area out "square" which will allow a proper scarf. The joint does not need to be perfect--using thickened epoxy will allow some deviation form a perfect fit.
