Hello jwingard,
We're unable to restore the Standard Master form. You'll want to do the changes manually.
- In the Custom form styles window, look for the Standard Master form.
- Click Edit and go to each tabs or sections.
- Save the changes and click Done.
You can also use this article for reference: How to Customize Invoices, Estimates, and Sales Receipts.
I'll be right here if you need anything else.