You can use a SceneCapture2D placed in your level, or SceneCaptureComponent2D as a component of some actor, rotate it so it’s forward vector is pointing down(pitch -90). Set it to orthographic projection. Set its ortho width to the size of the area of the level you want to capture and move it as needed. Keep the Z value of the orthographic plane in mind, as any object clipped by it will not have its backfaces rendered unless it is using a two sided material.
Create a Render Target, and set it as the Texture Target of the SceneCapture2D. Create a material, add a Texture Sample, set the Texture to the Render Target. You can then apply this minimap material to objects or your UI. As default SceneCapture2D will update every frame, but you can disable this and update it through a blueprint as needed.