This is a pretty good walk-thru on how he did it and the challenges he faced.
Using a tool created by another developer, Lucas Cardellini, Reubs exported the map of Springfield from the original game into Unreal Engine, then used a combination of AI and manual effort to add textures. He even went as far as to enable ray-tracing, and ripped audio from the original so the characters could speak.
For more, check out this Nintendo Life story.