Hi bartosh, for the face in your example there I'd rebuild it by first selecting this edge shown in red here:



Then run the Extrude command and snap the point to extrude to onto this endpoint here:



That will make a surface covering an area like this:



In order to incorporate the notches you'd then use the Edit > Trim command to trim the extruded surface by these edges here:



Now you'll have a trimmed surface that matches the hole and you can use Edit > Join to join it in and make the object a solid again.

There is some more information about techniques like this in the object repair tutorial here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

But often times problems in a particular face can be in the trimming boundaries and the underlying surface is actually ok. So instead of deleting that face like you show above it would be good to first extract that face and untrim it because that will give you the full surface and you won't have to remake that part. Then you can retrim it. That process of extract, untrim, and retrim is covered in that object repair tutorial.

At any rate, anytime you have a boundary with more than 4 edges, the technique for filling it is going to be to initially build an extended 4 sided surface and then use trims or booleans to trim away areas of it.

Hope that helps - if you have another example that you have a question about please also post the 3DM file so I can use your file to help illustrate what to do.

- Michael