The bulkheads are structural in that they hold your rig up, not because they stiffen the hull in any way. Starboard might work if it were thick enough, but you MUST fix those leaking chainplates first and foremost. If they aren't leaking, plywood bulkheads will last forever, leaving no need for alternative materials. If they ARE leaking, you will soon have much bigger problems than bulkheads.
As for the chainplates themselves ... I believe in removing them, thoroughly cleaning out the holes in the deck, put tape on the underside of the holes, then fill them with thin, slow-curing epoxy for a few minutes. This allows the epoxy to soak into the plywood deck core, making it resistant to future damage caused by inevitable future leaking. Then reinstall the chainplates with bedding compound of your choosing.
It's a pretty cheap and easy job, as boat projects go.