There is nothing to it. I once made web.py-server that combines maps from two countries. While kayaking. It was too annoying when you had to switch programs to see what is on other side of river. Unfortunately they do follow Google tiles standard anymore.

Originally this program was running on Python For Symbian, which is historic and funny. https://github.com/timonoko/webpy_map_server

Tiles.py seems to be how you decode other systems to the Google-tile system.