If you want to load a different portal.min.js file you can do so by modifying your Ghost config.environment.json file.
The default config is:
"portal": {
"url": "https://unpkg.com/@tryghost/portal@~1.1.0/umd/portal.min.js",
"version": "~1.1.0"
}
You can add this and change it, if you want to load locally or a forked version.