This is just a super simple example how to get started creating your own metadata mapping plugin for the REST OAI-PMH Drupal module.
In this demo there is also an example how to override a metadata mapping plugin provided by REST OAI-PMH. In this module, MODS is overridden. The relevant code can be seen here https://github.com/joecorall/rest_oai_pmh_jpcoar/commit/e32933a87915bf17515e58d8d29933b6df2fce8d