Even easier.
In inc_header.asp change "Username" to "Username or email address" for the login box.
In inc_func_common.asp, the chkUser function change strSql = strSql & " WHERE " & strDBNTSQLName & " = '" & ChkString(fName, "SQLString") & "' "
to strSql = strSql & " WHERE (" & strDBNTSQLName & " = '" & ChkString(fName, "SQLString") & "' or M_EMAIL = '" & ChkString(fName, "SQLString") & "')"
Your users should be able to enter their username or email address and password to login.
Totally untested so you may have errors.