Hello,
Just install Windows 10 and see if it will activate. I have been using Windows 10 on my Surface Pro (1^st^ Gen). I just installed windows then when I went online it activated.
You can go here for Windows 10: https://www.microsoft.com/en-us/software-download/windows10
After you install Windows 10, windows update will install the drivers you need.
Or if you want to stay with Windows 8.1, you can go here to Download a recovery image for your Surface: https://support.microsoft.com/en-us/surfacerecoveryimage