Water Damage - Touch screen on iPhone not working and black screen?

Hello! I dropped my iPhone 4 about 5 months ago in water. So now I want to bring it back to life. When I charge the iPhone it turns on and makes the sound when it boots up. However I am not being able to see the video, but a black dim light is produced. Also the touch screen does not work since I can't unlock the phone. All I seem to be able to do is call on Voice control and tell it to do tasks. Please help me, I have already taken it to someone who might be able to fix it, and they told me the motherboard had corrosion from the water. But I really know that I don't have to change the whole motherboard, I'm sure it is one or two parts that need to be changed. Thank you! This is such a great website!