The code required for the kernel is available, but proprietary modules are not. Thus you need to grab the latest stock firmware and extract all proprietary files needed, then build a device tree and build. If you're going to use the exact same TWRP build from them, you *may* be able to re-use the kernel, but you again need the proprietary files...and they may not like the kernel. That's why I mentioned you may need to just build the kernel. Don't use any unofficial ROMs unless you're extremely familiar with verifying the proprietary files, you might be locking yourself into their modifications to proprietary files specific to their ROM. Hair-pulling, I know. :/