Extracting json report info to a helper class

I have several json reports that are being generated in the following format, with similar header information but the returned data lists are of different custom types.
I want to write a generic helper class that can be used to replace all the individual header information, the majority of which is the same.
I also want to be able to pass in different data lists for the different reports. So creating a set helper class is no good.

e.g.

The report code:

public static CreateReport1 CreateHeader(List<CustomLevelReport> reportData)
{
CreateReport1 newReport = new CreateReport1 ();

newReport.TotalQuantity = reportData.Sum(x => x.Quantity);
newReport.TotalNet = reportData.Sum(x => x.Net);
newReport.TotalVat = reportData.Sum(x => x.Vat);
newReport.TotalGross = reportData.Sum(x => x.Gross); //<-- This header info mostly repeates for each report

//Then the report data is added
newReport.data = reportData //<-- this List type forms the body and would be different for each report

return newReport;
}

Instead I would like to call a hepler class for each report that automatically lists out the header part if not the data List, but I would like to pass that is as well, if possible.

e.g.

public static CreateReport1 CreateHeader(List<CustomLevelReport> reportData) // This code needs to be generic not fixed to each report tytpe
{
CreateReport1 newReport = new CreateReport1 ();
newReport = helper(reportData);
return newReport; <-- Returning the report with the header info with the specifyed List type data.
}

Is there a simple 'best' way to do this, giver the input List would not always be of the same type?