If you want a smooth surface, I would find the edges that bound the surface, then delete the existing faces and rebuild the surface. Or you can create a cutting surface and trim out what you don't want.
Hopefully you can find edges that will create a closed boundary. If not, you'll have to build out those edges as well. You may also need to create some guide curves to help the surface transition across. If you post the model and where you are wanting to replace faces, I'm sure someone will take it on and show you how to do it. I'd be happy to do it if my 9:30 call doesn't call me back.
Try the filled surface command.
Select the boundary of one half of this surface. Then select many (but probably not all) interior points in the "constraint curve" part of the Filled Surface feature.
Next, either mirror the piece, or create a new filled surface for the other half. I suggest using the existing edge of the mirror for one of the constraints of the second surface, and setting those edges to "tangent".
If you post the .sldprt I could show you how to do this very quickly.