Event Reporting to call out Custom Fields in related Contribution Record
If you have a report that doesn't show the custom fields on an entity, it might be because those fields are not configured as searchable (yes, a very non-obvious thing). If that's the issue, then it's the same as this question/answer: Custom fields in the Report Templates

Alternatively, if it's a problem with the report itself, you can generally hack it with something like the below at the top of the class file:

 protected $_customGroupExtends = array(
    'Contact',
    'Individual',
    'Contribution',
  );


If you're seeing some custom fields but not the ones for Contributions, just adding in the Contribution entry to your current values should work.