Rather than modifying a private copy, the best thing to do would be to discuss how you plan to integrate the two plugins with the current maintainers, and try and make your changes in such a way that they can be included in the public plugin. That way it will be easier to keep your changes in sync with future versions of Moodle and of the plugins, since the maintainers will be aware of your code and be able to avoid breaking it when they make changes.
General developer forum
Combine plugins together or build from scratch?
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