There are 26 system SSRS reports (not counting subreports) that come with Dynamics 365. If you find one that is close to what you need (like the quote report), should you modify the system report?

I say no. Here is why:

• System reports currently in Dynamics 365 are the same system reports that were in Dynamics CRM 3.0 (for the most part). They were written by someone who is probably no longer at Microsoft. When you dig into the RDL you will find that the most basic system reports include a lot of overhead and things like system reporting parameters that are not documented in the SDK.
• Even though the system reports have not changed much over the years, upgrades will sometimes overwrite/republish system reports. So if you modify the system reports, there is a possibility that a future upgrade could overwrite your changes.
• If you are online, you will find that some system reports still use T-SQL queries, while that is not allowed for us mortals. Microsoft sometimes gives themselves special dispensation to to things that we can’t. So if you want to copy the report and modify it, you will likely need to convert your copy to FetchXML.

For these reasons, other than minor/cosmetic changes to reports, my recommendation is do not modify the system reports. A good SSRS report writer can recreate the system report faster than she or he will be able to decode what somebody who is no longer at Microsoft was doing 10 years ago.