Swift iOS App - Built with Concrete Classes, Views and ViewControllers to be abstracted for client-by-client overrides

This is a conceptual question regarding an App that my team has nearly finished in the most top-down, concrete way possible. All the features work, it's being polished to be published, but here's the rub:

We will white-label the App and publish ~20 versions of it, some clients requesting customizations to certain Views and View Controllers, other clients requesting customization in other Views and VCs. So I need the ability to override particular Views or ViewControllers (and if possible be more granular and override a single function on a VC).

I think the approach is submodules per client, with a folder structure like:

1. system/ - core project files
2. apps/ - client specific files (git submodules)
3. apps/client1 - example client folder with client specific overrides

So, if I have a class like RootViewController.swift, that has particular methods on it and functionality, including instantiating several UIViews, how could I build an override system to look in the client's override folder for overrides, if does not exist, use system level.

Really appreciate any help or guidance in this.

Cheers, Rob