Please add support for Windows 8.1 in DownloadUpdates.sh.
It seems as if it is prepared, as you can find indications for w63-x64 and w63, but it is not implemented. The menu does not provide a way to select it. If you provide command line arguments for w63-x64 it quits with "system w63-x64 does not exist."
If you select "[12] All 64 bit" from the menu then it tries to download w63-x64, but fails with the same error.