The plastic sheeting is there only to protect the foam from resin fumes during the manufacturing process. I would not recommend removing the old foam unless it is severely waterlogged or breaking up.

The foam is the only thing that keeps the boat afloat in the event a hull fills with water.

What's wrong with your existing rudders? Pick the one that works the best and use it as a template to drill the other one after filling the old holes in it. (Use a drill press to make sure the holes are drilled straight through.)