Here's another way to do it.
Start by going into Edit Mode and selecting all the inner edges for the hole in the visor. You might have to Shift + Alt + click a few times to get them all selected.
Press Shift + d to duplicate these edges only. Press g to grab them and pull them away from the main mesh so that you can confirm that they did indeed duplicate.
Press Numpad 1 to go into orthographic front view. With the new edges still selected press s to scale these edges up until they look about right for your purposes. You may have to scale the different axes individually, or even adjust some edges individually. But not too much tweaking is needed. Then press p and from the Separate menu, choose Selection. Now your edges are a separate piece from the main mesh. This is important for the next step. So is the front view. Make sure you're still in orthographic front view.
Go into Object Mode, select the new "edge" object, then Shift + select the base mesh. Now, with them both selected, go into Edit Mode. They are now both accessible in Edit Mode. Go to the Mesh menu at the top of the screen and choose Knife Project. This will use the edge object to cut a new set of edges into the main mesh. There is a special checkbox setting for this tool called Through. It's accessible from the panel to the bottom left. By default, it is unchecked, and that's what you want. If it were checked, the cut would go through both sides of the main mesh, instead of only the front side.
Here is the result. You can delete the "edge" object after you are done doing this. You don't need it anymore.
Here is what it looks like after a few quick cuts with the Knife tool to clean up the topology and get rid of the ngons.