Creating extension with civix: How to use CRM dir?
Overriding core files was how we used to make changes until collectively we figured out that was storing up trouble for the future.
Another possibility is that if you think this is likely to be a small, useful improvement for most users of this aspect of CiviCRM then you could propose it as a change for core - go here and use the 'improvement' template.  If that is agreed then submit your changes as a PR.