Where are the chip configurations stored in IDE 1.6.x? How to modify them?
You are importing a json file provided by someone for free.
The best thing you can do is contribute by adding ATTiny13 to this repository.
To do so:
So go to github (create a account if you do not have one);
create a fork of the repository.
Add your ATTiny13 board to the boards.txt file.
mod the json file to contain a new version and the attiny13.
Test and if everything works fine
Create a pull request