Custom maps for Map Report

Is it possible, for the Map Report, to use a custom base map instead of Google Maps?

Yes, the following article describes that it is possible to change the default maps engine used by Survey Solutions for the map report, and demonstrates how to specify Google maps for China as an alternative:
https://support.mysurvey.solutions/headquarters/config/alternative-maps-for-servers-in-china/

But, the new engine will need to be 100% compatible with the Google Maps API.