Top case water damaged, do I have to change motherboard?

APPLE MACBOOK PRO 13" A1278 2010

It has water damage at the keyboard and touchpad recently. The keyboard and the touchpad are acting strangely. I could use an external keyboard connected to the usb port and it would act correctly; however, an external mouse would't help the touchpad issue (does this mean the motherboard is partly broken??). So I am wondering if this is the only part I should buy to fix the problem (just to replace the whole top case) (I found this on ebay), OR do I need to buy another motherboard and the top case?

Thanks!