Hi Tilo,
What I would recommend doing is follow my tutorial on Customizing Widgets (https://theeventscalendar.com/how-to-completely-customize-widgets/) but do the customizing for the TribeEventsListWidget found in plugins/the-events-calendar/lib/widget-list.class.php – I would create another separate copy of the widget (still keep the old one).
Then, all you need to do is change the query there so line 43 reads:
$posts = tribe_get_events( 'eventDisplay=past&numResults=' . $limit .'&eventCat=' . $category );
instead of:
$posts = tribe_get_events( 'eventDisplay=upcoming&numResults=' . $limit .'&eventCat=' . $category );
Hope that helps.
– Jonah