The Debugging tools for Windows (a.k.a. Windbg) for Windows 8.1 can still be downloaded separately. Although it's now part of the Windows Driver Kit (WDK), you can still download the Windows 8.1 SDK Installer and select only the debugging tools. It's an about 180MB download (without the symbols).

http://msdn.microsoft.com/en-us/windows/hardware/hh852365