Aggregator Summary Item Theming

I'm working on aggregator-summary-items.tpl.php to create a custom .tpl.php file for the aggregator summary items, I'd like to change what data is actually being pulled though in the $summary_list variable.

It looks like I can edit theme.inc as per this page http://api.drupal.org/api/function/theme_item_list/6 but ideally I would rather not. Is it possible to access/use some of the variables from aggregator-summary-item.tpl.php? Basically, the items I need to use are:

$feed_url: Link to originating feed.
$feed_title: Title of feed.
$source_url: Link to remote source.
$source_title: Locally set title for the source.

What do I need to do in order to make these accessible in a custom aggregator-summary-items.tpl.php file?

Thanks