Can the gelatin be subbed out with agar agar or something else for a vegetarian recipe? Would you add a different amount?

https://onolicioushawaii.com/agar-agar/
at the end, this gives ratios and additional information

Stef - the short answer is yes.
This recipe sounds delicious.
But/and because jelling agents work differently, I wouldn’t spend time tinkering with this one.
There are vegan versions of this dessert using agar-agar.
Suggest you locate and use one of them.
Enjoy!