I tossed this my Zen Cart directory in a file called product_info.php ...
<?php
// osc to Zen Cart redirect
// $Id: product_info.php 1591 2005-07-15 19:57:46Z ajeh $
//
$_GET['action'] = '';
$_GET['main_page'] = 'product_info';
require('includes/application_top.php');
$destination = str_replace('&','&',zen_href_link(FILENAME_PRODUCT_INFO, 'products_id=' . $_GET['products_id']));
// the 301 in this header() command tells a spider/browser to re-learn the URL (301=Moved Permanently)
header('Location: ' . $destination, true, 301);
?>
That should handle products ... or send them to the missing product page in your Zen Cart site ...