Water damage repair

I have water damage to the passenger side sidewall on both the inside and outside. The wall is somewhat distorted. The outer skin seems to be loose from the underlying wood structure.

Water got in from the mirror mounting screws. The mirror was week and bounced when driving. When I removed the mirror the screws were wet and rusted. The mirror bounch was caused by the weekened wall structure that the mirror was screwed into. I drilled new mounting holes and used nuts on the inside to add support for the mirror.

Is it possible to replace the underlying wood ? I don't see any easy seems to work from on the outer skin around the area above the wheelwell.