Import the scan in MeshLab

Create a mesh using Filters -> Remeshing, Simplification and Reconstruction -> Screened Poisson Surface Reconstruction. Export the mesh as .ply

Import in Blender and scale/rotate to fit

Go in Edit Mode (Tab), Select Face Mode and Select by Circle (C) faces around the opening to fill. Increase/Decrease the circle size with middle mouse button. Press Esc to pause the selection and to navigate the model.

Press Shift+D to duplicate the mesh faces and Esc to position in place. Press P -> Selection to separate the selected faces from the main object

In Edit mode, Select Edges Mode, press Alt+Left Mouse Button twice on an edge to select all connected edges. Then press P and Selection to separate the loop

Select all Vertexes, then from the Vertexes menu select smooth

Select the edges and press F (Fill)

Result