public class ReportHeader : ReportSectionElement, ArcGIS.Desktop.Mapping.IElement, ArcGIS.Desktop.Mapping.IElementContainer, System.ComponentModel.INotifyPropertyChanged, System.IComparable, System.IDisposable, System.IEquatable<Element>  

//Get the "ReportSection element"
//ReportSectionElement contains the ReportHeader, ReportPageHeader, ReportDetails. ReportPageFooter, ReportFooter sections.
var mainReportSection = report.Elements.OfType<ReportSection>().FirstOrDefault();

//Get the ReportHeader
var reportHeader = mainReportSection?.Elements.OfType<ReportHeader>().FirstOrDefault();

//Get the ReportHeader
var reportPageHeader = mainReportSection?.Elements.OfType<ReportPageHeader>().FirstOrDefault();

//Get the "ReportDetails" within the ReportSectionElement. ReportDetails is where "fields" are.
var reportDetailsSection = mainReportSection?.Elements.OfType<ReportDetails>().FirstOrDefault();

//Get the ReportPageFooter
var reportPageFooter = mainReportSection?.Elements.OfType<ReportPageFooter>().FirstOrDefault();

//Get the ReportFooter
var reportFooter = mainReportSection?.Elements.OfType<ReportFooter>().FirstOrDefault();