You can’t change those responses from Airtable’s API.
If you want to create your own custom responses, then you would need to setup a scenario in a webhook automation platform like Make.com.
Setup a webhook to receive your GET call, perform your Airtable search, and then setup a router with 2 different filtered paths (i.e. conditional paths) based on the results of your search.
You can then customize a different webhook response for each conditional path.
Here’s a screenshot of how this would look in Make.com — it’s very easy: