Hi,
There is no HTML view in the plugin. The HTML of the notification box is generated in javascript on the fly by the browser.
So it's not possible to provide a view to override.
And regarding the duplication of the plugin, the add to cart notification system has been made especially so that it's possible to create notification plugins in a standard way.
As long as you implement the javascript trigger from HikaShop you can develop a plugin independently from how HikaShop evolves.
So duplicating the plugin to make your own custom won't be a problem even if we change methods or functions, because you'll only rely on the javascript trigger, and this one won't change.
Also, most of normal customization of the notification box can be done either with translation overrides or CSS code.
It's only if you want to add extra HTML elements to it that you need to modify the javascript of the plugin since the HTML elements are added dynamically by the javascript.