I think that the easiest and cheapest solution would be to install a raised, tightly constructed wood floor which would solve the problem of the decayed boards at the bottom. Holes in the wall can be covered with hardware cloth - windows! If there are spaces under the eaves between the roof and walls, they will need to be covered too. The end result would not be scenic, perhaps, but it would be made safe from predators. Add roosts and nest boxes and the chickens will be content.
