Depends on the exterior of the foundation. If it is or will be faced in brick, you could centre the door, add a couple of studs on each side and use wider casings all around. Same thing if it's to be concrete only. Or possibly use a column of concrete blocks cut to size and parged over. Assuming you're parging over the rest of the exposed foundation.

Not too neat in their cutting were they ? You can tuck point and fill the holes / lines your self. Rinse the area of excess dirt / dust with water. When almost dry, fill the holes with mortar / cement, pack them solid. and make the cement flush with the block. When the cement is becoming hard but not all the way hard take a damp sponge and lightly rub over the cement to make it look uniform like the block itself.