Hey Gavin,
Thank you for reaching out to us!
Before we begin, just to set expectations, the scope of our support is mostly to get our customers started on the right track and to help them in case of issues. We unfortunately do not provide complete support for customizations like this one.
To help you get stated, you can create a template override for the Month Single Event file on the-events-calendar/src/views/month/single-event.php and use the tribe_events_count_available_tickets function to achieve what you want. Here’s an example of what you can add around line 197:
This event has tickets available
You’ll find more information about template overrides in the following article:
https://theeventscalendar.com/knowledgebase/themers-guide/
I hope this helps! If you have any other questions at all please feel free to let me know and I’d be happy to help as best I can!
Regards,
Patricia