You can always get byondexe from the builds directory.

http://www.byond.com/download/build/509

The files missing include the ones that were mentioned in the install.nsh changes. Install.nsh itself has been updated in the newer versions to reflect that and other changes, and as of 509 there are now DirectX files involved too.

The files you'll need to update are all the bin files, install.nsh, and byondexe.exe.