Try putting a text box on the form and setting its value property to "X" when the value is true. You will also need to put the yes/no field on the report, but you can set its visible propery to 'No'.
If you are using Access 2000 or higher, you can use the Report_Format event:
If Me.YourYesNoField = True Then
YourTextBox.Value = "X"
Else
YourTextBox.Value = ""
End If
Good Luck
-Gary