You'd need to customize a few things ...

In the case statement add:
Code:
// bof: add products_model
    case('products_model'):
        $dir_model = $op_dir;
        break;
// eof: add products_model
Code:
                <td class="dataTableHeadingContent" align="center"><a href="./stats_inventory_report.php?page=<?php echo $_GET['page']; ?>&sort=cd.categories_name&dir=<?php echo $dir_name; ?>&cat=<?php echo $cat; ?>"><b>Master Category</b></a></td>
<!-- // bof: add products_model -->
          <td class="dataTableHeadingContent" align="center"><a href="./stats_inventory_report.php?page=<?php echo $_GET['page']; ?>&sort=products_model&dir=<?php echo $dir_model; ?>&cat=<?php echo $cat; ?>"><b>Model</b></a></td>
<!-- // eof: add products_model -->
                <td class="dataTableHeadingContent" align="center"><a href="./stats_inventory_report.php?page=<?php echo $_GET['page']; ?>&sort=products_quantity&dir=<?php echo $dir_quantity; ?>&cat=<?php echo $cat; ?>"><b><?php echo TABLE_HEADING_QUANTITY; ?></b></a></td>
Code:
// bof: add products_model
  $products_query_raw = "select p.products_id, products_quantity, pd.products_name, p.products_model, p.products_price, (products_quantity * p.products_price) as total, categories_name from ".TABLE_PRODUCTS." p left join ".TABLE_PRODUCTS_DESCRIPTION." pd using(products_id) LEFT JOIN ".TABLE_CATEGORIES_DESCRIPTION." cd ON(cd.categories_id = p.master_categories_id AND cd.language_id = '$lang_id') $db_category_where group by p.products_id order by $sort $dir";
// eof: add products_model
Code:
                <td class="dataTableContent" align="left"><?php echo $products->fields['categories_name']; ?>&nbsp;</td>
<!-- // bof: add products_model -->
          <td class="dataTableContent" align="left"><?php echo $products->fields['products_model']; ?>&nbsp;</td>
<!-- // eof: add products_model -->
                <td class="dataTableContent" align="center"><?php echo $products->fields['products_quantity']; ?>&nbsp;</td>
Code:
              <tr class="dataTableHeadingRow">
<!-- // bof: add products_model -->
                <td colspan="6">&nbsp;</td>
<!-- // bof: add products_model -->
                <td class="dataTableHeadingContent" align="right"><?php echo $currencies->format($total); ?></td>