Do what @casiepa said and just put the bbPress template content-single-forum.php in a folder called bbpress in your child theme, and edit the file.
A guide like this would help you figure out how you can customize the bbPress default theme in a child theme.
https://codex.bbpress.org/themes/theme-compatibility/
Just make your file look similar to this gist.
https://gist.github.com/robkk/059c1f87b3d21cf75dd9
@tech55541 thanks for helping out in a few topics in the support forums, but your code shows undefined messages in the user profile subscriptions section if you are subscribed to a forum.