Hello Colin
The nose cone on the M’s is made of steel. If yours has been cut out as in the picture above and you want to fill it back in so it is more like an original, there is really only one way to approach it to make it work. A piece of sheet steel is needed about 1 mm thick to cover the opening that you are trying to fill inn. You should allow at least 1 inch beyond the opening for trimming . The dimensions you need will come from the nose cone opening itself .
The nose cone opening in the area that’s under the grill will have to have its edge joggled , this will create a stepped lower edge for your new panel to sit on. The new panel can then be trimmed to match the lower edge and spot welded in place.
When all looks correct the outer joggled edge seam can be filled in with lead or filler to make a smooth appearance on the outside. This is not a complicated task but it does require some special tools to achieve it. I have attached some pictures to give you an Idea of what to do. My nose cone was quite bad with rotted out sections but the principal is the same, its just adapting it to your needs.
I think you will need to get some help in doing this work but the end result will be worth while. Let me know how you get on and if you need any other guidance.
Kind regard, Charles