I have not personally tried this, but the following link provides what SEEMS to be a way to get a Windows 8.1 ISO for your specific product key Please correct me if a I wrong about this as I do not want to mislead anyone.) Either way, make a full system image of your present setup BEFORE you try this:
The link offers two Options:
Option 1: To Download a Windows 8 or 8.1 ISO File from Microsoft
Option 2: To Create a Windows 8 or 8.1 ISO File from "C:\ESD\Windows" Folder
Note also, that the following "interesting" method of getting the ISO file was posted here:
"This will allow you to download the official Windows 8.1 ISO from Microsoft.
What you do with it there after is up to you.
Note that Windows 8 to Windows 8.1 is done (as far as i know) via the Windows App Store. This i find annoying as i like many of you like to perform clean installs.
My senario went like this...
I upgrade my Windows 8 to 8.1 via the store, i did this thinking that i could 'Reset' my PC and thus get a clean install while maintaining the activation etc.
That did not go to plan. I received an error after clicking on the 'Reset' informing me that there are missing files, please insert media (or something like that).
Well guess what... i don't have the media.
So i go download the Windows 8.1 Setup and try, to no avail, to enter my Windows 8 product key.
Follow these steps to download the Windows 8.1 ISO.
1.Go here Upgrade Windows with only a product key - Microsoft Windows Help scroll down and click on both 'Install Windows 8' and 'Install Windows 8.1'
2.Run the Windows 8 setup and enter your retail Windows 8 key, click next and when i begins downloading, STOP it! Exit setup.
3.Now run the the Windows 8.1 setup. It will produce and error. Exit and then run Windows 8.1 setup again.
4.It should now be downloading the Windows 8.1 ISO. Afterwards you can save it as an ISO or create the USB media immediately.
You can use this to 'Reset' your PC by booting into setup and choosing repair my PC. Follow the on screen prompts for resetting your PC."
My comment: I am not sure that this will work for a CLEAN install!! (A believe a new Product key would be required(?))