wood stringer rot

[yanknbank]

After pulling up the floor on my new to me m5 on preparation for annual, I'm finding the belly stringer is warped in some areas. It looks great through the tail area, but it must have seen moisture in the insulation. It's original razorback. I anticipated a recover in the coming years, but I'm not ready for that yet. what level of degradation would you consider to be unairworthy for these? Can they be treated with a product to try to penetrate into what remains? Maybe a fiberglass resin or wood hardener?

[maules.com]

sometimes its possible to slide a doubler in beside the original and keep apropriate tension on the fabric.
make sure the drain grommets are open.