Project File Help! (Should be easy)

Question

Im not well aware with Illustrator but I believe the community here should be able to help.
I need help breaking off the part circled in green to match the original file. I used the ellipse tool along with some pathfinders to get to where Im at,now I'm stuck as to how to break it off.
Any ideas?
Here is a screenshot of my work space if it may help..

Answer 1

Ashab,
Another approach, for greater/full accuracy of especially the semicircular parts, working on top of the locked original, starting with stroked paths (no fill) where the Stroke Weight is identical to the width of the original final paths (Smart Guides are your friends):
1) Create four full concentric circles with the Ellipse Tool corresponding the more or less half circles;
2) ShiftClickDrag with the Line Segment Tool between the outermost Anchor Points of the largest circle, then Object>Path>Divide Objects Below, then delete the lower halves;
3) Lock the smallest circle and ShiftClickDrag with the Line Segment Tool across the second smallest circle where it is to be cut (four places symmetrically) and delete the three unwanted bits; this will give the right angles of the cuts;
4) Create the straight bottom path half a Stroke Weight under the half circles so the strokes touch:
5) Select everything and Object>Path>Outline Stroke to get closed filled unstroked paths;
6) Lock the two smallest circles and ShiftClickDrag with the Line Segment Tool across and beyond the two largest circles in both sides where they are to be cut, then Object>Path>Divide Objects Below, then delete the lower unwanted bits (three out of four), then select the second largest circle and the remaining bit and Pathfinder>Unite; this will give the right angle here;
7) Select the straight bottom path and the relevant circle parts (just ClickDrag across with the selection Tool) and Pthfinder>Unite.
That should do it.