iPad 1 touch screen or motherboard

My son saved his money for a year to buy an iPad and found one with a broken LCD. To make a long story short, the LCD replacement works flawlessly, but the touch screen is not responsive. Everything else works -- it can sync and restore, play from my car (Ford Sync via USB) and everything else. The only thing is the touch screen does not respond. I've tried every suggested recovery technique -- reset, restore, update, but no joy. Given that we got it so cheap, I don't mind shelling out a few bucks to help repair it, but I need to know what to repair. The original owner said he dropped it and stepped on it, which broke the LCD and bent the body a bit but didn't break the glass on the touch screen. So is there any way to know if it's the screen itself or the motherboard that was damaged? Or should I just not worry about it, buy a new MB and be done with it?