Modifing core or contrib modules is something you should avoid. It will cause problems or at least tedious additional work when you want to update.
If the module does not offer any possibilities to theme the output to your needs I'd sugest to add a feature request in the modules issue queue (check whether it does not already exist). Then go and make the output themeable and add your changes as patch.
This involves getting familiar with drupal theming for module developers and the drupal contrib developement process which could take some hours. But you'll win in the long run. If you're patch is accepted the feature will be part of the module this free's you from maintaining and mergin in your changes for every update.