Hi,
I suppose with ‘core’ you mean the bbPress plugin code ? NO, NEVER ! You don’t want to loose your changes at the next bbPress update, right ?
I think @robkk answer was the most appropriate one for this topic: Just work on the template, copy it to your (child)theme and modify as he indicated.
See probably step 5 and 6 from https://codex.bbpress.org/themes/theme-compatibility/step-by-step-guide-to-creating-a-custom-bbpress-theme/ an also https://codex.bbpress.org/functions-files-and-child-themes-explained/
Pascal.