With transformation rule feature you may modify the clone of the source website that you've chosen to use as a Safe Page.

To do that you need to go to Websites > Domains > Edit Domain.

Scroll down to the advanced settings and click "Add new transformation".

Once you've added a new transformation, you can use the template to type in the changes that you want to be applied to your Safe Page. You may check the changes once you've created a campaign with that domain and been filtered to the Safe Page.