You could go for a fresh install of Windows 10; it uses it's own compression to leave with as much (possibly) more space than you'd get with WimBoot on Windows 8.1. From what I've read the activation key is built in to the BIOS so provided you've not messed with that Win10 should activate.
Alternatively if you have access to another Windows 8.1 PC you can create your own WimBoot System relatively easily, there is a guide in the guide section. It does take more work but isn't too difficult. There is also some commercial software available that will create a WimBoot type environment from a standard install which will give you back the same amount of space you would have with the factory fresh WimBoot.
If you go with any option that involves a fresh install you will need to download and manually install the drivers and you will need a keyboard.