Ionic.config.json type=custom missing config for ionic1 running ionic7 cli

So currently im wotking in upgrading an ionic1 app to use the latest ionic 7

That been said the option for type “ionic1” was deprecated at ionic6 but saw that theres an option to leverage type custom, the thing is that i couldnt find anything regarding how can i use it and all i have in the console is “provide ionic:serve script” so far i tried looking for old build and serve ts files that got removed but havent been able to figure out how to doit without downgrade the cli

Thabks in advance