If I wanted to modify the Narrated Website Report, I’d probably start by cloning the Built-in files and subsection of the registration file as an Addon with a unique ID.
That way, the regular updates that @SNoiraud posts to the built-in would not overwrite my experiments. (The 5.2 update has an extensive list of Narrated Web Site Report changes you could review.)
If you are talking about the skin of the Reports, you’d probably start with changing CSS. There’s another thread about that: “Narrated Web Site StyleSheet doesn’t apply”
If you are looking at changing content, you’d want to start by looking at the GitHub Pull Requests and MantisBT issues related to this Narrated Website Report.
Reviewing the objective Serge targets and the code changes necessary to implement is a good onboarding task.