If it's rusted enough to weaken the steel, which it sounds like it is, those sections will either have to be replaced or patched/ reinforced. Patches usually are visible and not very attractive, but easier than replacement. As for finding someone to do this kind of work, you may have to do a lot of looking. I would start with welding and fabrication shops, esp. if you can find one that does decorative wrought iron work. A plain old industrial type welding shop will probably be too heavy handed- you need someone who understands this type of work. Make sure whoever you hire understands what you want, and gives you a firm price. Also realize that this kind of work may take a lot of time, and welding shops aren't cheap. They have a lot of expensive equipment, extensive training, and overhead to pay for.