As so often with elements defined by sketchlines, you can't get the sketch direcly, but you have to use the Temporary Transaction Trick to get access to the sketchlines.
StringBuilder sb = new StringBuilder();
Element elem; // MatchLine - element
List<ElementId> deleted = new List<ElementId>();
using (Transaction t = new Transaction(doc,"dummy"))
{
t.Start();
deleted = doc.Delete(elem.Id).ToList();
t.RollBack();
}
List<ModelCurve> sketchLines = new List<ModelCurve>();
foreach(ElementId id in deleted)
{
Element e = doc.GetElement(id);
if (!(e is ModelCurve)) continue;
sketchLines.Add( e as ModelCurve);
sb.AppendLine(string.Format("<{0}> {1} {2}",id,e.Name, e.GetType()));
}
TaskDialog.Show("debug",sb.ToString());