Missed light key, walk (c) key, Self Delete combination and perhaps a enter key. All could be used on D-Pad vertical/horizontal axis.
Then you could assign the D-Pad diagonal axis on numbers 1-4 for building. While this would not really fix the fact that you wouldn't be able to build well at all, you will be able to build bricks 1-4 as long as you place them correctly from the beginning (otherwise just keep placing them until they work) and place them with the D-Pad enter.
In another scenario, maybe there could be a way to "toggle" between building and playing mode, in which case you could take advantage of having more buttons availible for building? You could have the back button switch between toggles, if that would be possible.