I recommend manually moving all the 'old' system items into a backup folder (just in case), then using a tool like
Carbon Copy Cloner to copy the system files from your FW drive. CCC is designed to take care of all the dark magicks required for blessing an OS X volume. It's served me well in the past. I've used it to copy system files without user files and apps many times.
Edit: Naturally, you should back up your data first. Just in case.