hello, you just need to replace the sim card reader, you need soldering skills, but it's not a difficult work if you have experience. search on eBay "nokia (model ecc) sim card reader" and you will see what I'm talking about. You DONT need to replace motherboard, its incredibly stupid.
Ok, water damage is usually solved with isopropyl alcool in most of the cases. By the way if the water has corroded some lcd pin its obvious the connector is permanently damaged. So you need to replace it. I don't know if you have experience with that so tell me if you want to replace it, i will give you some tips. bye.