Hi Justin.
The Related Events display/output is handled by this file: /wp-content/plugins/events-calendar-pro/src/views/pro/related-events.php
But it actually gets its events from tribe_get_related_posts(), located in /wp-content/plugins/events-calendar-pro/src/functions/template-tags/general.php
You could filter this function’s results (i.e. keep the same output/display) via tribe_related_posts_args or tribe_get_related_posts, or you could override the output/display (including using a function of your own) via our Themer’s Guide, in which case you could probably use tribe_get_next_event_link(), located in /wp-content/plugins/the-events-calendar/src/functions/template-tags/link.php
I hope this helps you with your customization!