I'd like to modify the sign up page to say

 

Email Address - Please use the same email address as you used on x to ensure you can access the restricted forums.

 

Can anyone point me in the right direction of which php/html files these assets are generated from? I can figure it out from there.

 

You mean like this:

Or in its own field (e.g. file location and edit file)?

If you are looking to simply add the text next to the Email label field follow these instructions:

GO TO

• Admin CP > Customization tab > Languages

SETUP

1. Once there click this button (located top right corner): & click it.
2. This will pop-up:
3. 
4. The "Quick Translating" will be off by default. Click it on and save.
5. Well logged in as an Admin on the Front side go to: yoursite.com/index.php?/register/ (I tested this and it works; hence why mine is clicked on & changed in the 1st post in the image above)

EXECUTION

1. When you mouse over TEXT it will highlight as yellowish:
2. 
3. Now PRESS & HOLD the LEFT mouse button until it changes to a text edit field.
4. it changes to an EDITABLE type box :
5. 
6. Type what you need (though I find, copy and paste better) and hit "Return/enter".
7. Then go back to Admin CP > Customization tab > Languages and turn "Quick Translating" off.

NOTE: Quick Translating can be used to edit most words that appear on the front end (e.g.):

I used it to change "FORUM" label on the index page & the breadcrumbs home link text.