In BeforeEventRender event handler, you can reach the original data item using e.DataItem property:
e.DataItem["field1"] // field1
e.DataItem.Source // the original object
e.DataItem is accessible automatically.
In addition, you can specify custom fields using DataTagFields property. These fields will be accessible through the full event lifecycle, i.e. in BeforeEventRender, on the client side (e.g. EventClickJavaScript) and in the server-side event handlers (EventClick).
DataTagFields="field1"