No keyboard, No trackpad, loud fan. Is it my Logic Board?

So my MacBook Air was found with liquid damage. My keyboard and trackpad do not work at all, however when I plug in an external mouse/keyboard it works. My fan is really loud and doesn't seem normal. I took it to apple and they said it costs $1200 AU. I might as well buy a new one. I would like to know if its the Logic board that is causing these issues or the top case of the mac that needs to be replaced.?