First, we should solve why id doesn't boot. What do you see on boot? What type of screen? Any icon like a folder with a question mark? What have you tried so far?
Second, the keyboard can be replaced. Unfortunately you will have to replace the top case on the computer. Upper case replacement guide here [guide|27438] . The parts for this are here [product|IF123-033]
So to replace a keyboard is expensive so solving the boot issue is probably a good place to start as the computer could be used with an external keyboard and mouse as well. Also what version of Mac OSX you are running would help(if you can remember)