How to "Reverse Engineer" mesh created out of smooth curves with a missing chunk? I have a model that needs a little mesh patching which has been imported from a .stl file. It seems like the model started off with a nice curving face made out of triangles, which was then cut through and patched to add features. The goal is to remove those features and bring the model back to its original curved face by adding/removing vertexs where they need to be. Below is a quick example as to what currently needs to be patched.



The selected hole will need to have the already existing sweeping curves reintroduced in order to look as it did before, as seen below.



I've tried to do a few methods in patching over the hole, such as bridging the edge loop, grid fill, and just adding a face over the hole, but it seems like the new faces created looked flat instead of following the contour of the model. grid fill may do the trick, but I may end up dealing with a miss count of vertices on all four sides of the grid layout. I believe shrink wrapping a subdivided plane over the hole worked pretty well, but causes plenty of odd looking geometry. Any thoughts?
You can fill holes in curved surfaces like this either with extruding with snapping set to Vertex or using Mesh:F2 addon.

Using F2

After enabling add-on, select vertex in the corner of the hole and hover mouse cursor over hole (mouse position matters regarding to where face is created if there are several possible ways).
After that press F to fill the face. Continue until finished with other vertices.
Split uneven parts of hole to make it of square / rectangular form. Once it is use Ctrl+F > Grid Fill and choose options to have clean topology.



See more about filling faces with F2 addon: Make quad face from where two edges will meet?

Extruding and snapping 

The same could be done with extruding with snapping turned on and set to Vertex as element to snap to. Fill the surface until the hole is rectangular and then use Grid Fill.