Scale the 2nd face to the same of the 1st face.Draw curve to the outside of the 1 st face then use mesh boolean split command and adjust it and then join it.
Would be a lot easier if you found a coin shape without a face already on it - there must surely be something like that on the internet. Or do you absolutely need to use that base shape?
I don’t work with meshes often so just used Shrinkwrap to close it - there’s other ways to close it but I wanted to get this done quick. Maybe someone else can advise on best way to close the mesh without needing to shrinkwrap.