I’m currently running the latest testing build of overviewer. It’s been going for about 12 hours, and says it has 31 hours to go. Once it’s done, most of the missing blocks should be restored.

Update: rendering complete (20180826@11:53, 27 hours rendering time), new map is live. Most missing blocks appear. The only anomaly I have noticed thus far is that rail is not oriented properly. Well done, Minecraft Overviewer team!