If what you are missing is in chunks, you could pull the chunk files from your unbuilt map. Dinnerbone created a web tool for you to calculate the coordinates to the chunks:
This isn’t always the easiest method, but it is the most rudimentary. You’ll just copy the chunk files from your unbuilt folder to your folder with missing chunks.
Edit- Actually, you could try copying all of your new chunks onto a copy of the unbuilt world. This should overwrite all of the chunks that a new version exists, and keep the remainders from the unbuilt world.
As long as your map uses Minecraft’s vanilla-normal blocks, you could drop the world into a spigot server, configure and use World Edit. World Edit has a restore feature that can pull blocks from a backup:
Set your unbuilt world as a snapshot backup, then use world edit command inside the game to restore areas.