I recently designed a large silo on a skirt, we ended up with two large opennings. My first approach is always putting back the material to provide the same section properties as per jte above, however we had concerns about the anchor bolts and transfer of loads so we added a central stiffening...
If the welds are still above the minimum required plate thickness then I would just clean them up and blend them in. If however they are below the minimum thickness I suggest you will need to build them up again. The vertical welds are the most important in this regard. The Horizontal welds can...