I’m afraid the keyboard is part of the upper case of the MacBook, meaning that you’ll need to get a complete upper case in order for you to fix the keyboard. You can read more in the [guide|135292|upper case replacement guide.] But I’m not sure if the membrane is that critical to the operation of the keyboard. Maybe try first if simply re-assembling the keyboard makes it work again, that way you don’t need to buy an expensive upper case.