Yes the keys can be replaced. But you must be gentle and careful when removing and replacing the keys as there are delicate plastic clips on the underside of each key which engage in the scissor mechanism beneath and are easily broken.

@aromarood - I had the same issue and was able to resolve it myself after a wasted trip to the “genius” bar to be told by a moron that I could buy a wireless keyboard. It is important that the space bar is both removed and replaced in the right manner to avoid breaking delicate plastic clips on the underside of the key and because one set of clips is designed to slide into the scissor mechanisms while the other side clips down vertically after you have engaged the other set of clips by sliding them in horizontally. It is much easier to demonstrate than to explain. In my first attempt I forgot to remove the lengths of wire from the underside of the key before replacing the key. When you pry the key off they tend to stay with the key, but you must set them in their correct positions on the base plate before you press the key back on or the key will not spring up when depressed - which sounds like the situation you have. In the image below you can see the wires still attached to the key. They are retained by...