In the core files there is a file called widgets.php inside /plugins/bbpress/includes/common
If you look at line 677 (in my version) there is a topics widget class (class BBP_Topics_Widget), you could create your own plugin and copy the class across (renaming it) and then create your own bbPress widget, without altering the core code.
Good luck!
Thanks,
It is only 1 change in the widget.php, so just changed the core file.
line 871 (bbPress2.5.1) bbp_topic_last_active_time to bbp_topic_post_date
We have an open ticket on Trac on what is and isn’t in the widgets and the potential option to merge everything into a single widget. If you have any additional thoughts not yet covered so far please feel free to add your own thoughts 🙂
https://bbpress.trac.wordpress.org/ticket/1918