Silhouette of imported curves/dwg

I am very new to Rhino and have been impressed with how much it can do. I have started using it for 2D work as well as 3D.
I am looking for a way to create a outline or silhouette from a drawing I have imported. It is a drawing from a manufacturer that has way to much detail for the scale of the drawing I am working on. Since this has and will keep happening I am hoping to find a way to quickly remove a lot of the extra details/line work. My first thought was to just create a simple outline similar to how the silhouette command works for 3D objects.

Any ideas for a newbie?
pump.3dm (4.9 MB)

CurveBoolean could be your friend here, explode - select all - CurveBoolean - click outside the drawing.
However. it is this kind of inaccuracies in the original drawing that will kill most efforts:

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