National 12 Dinghy Restoration

Hi I have recently looked at a National 12 which requires substantial repair/restoration. The boat is of marine ply construction and will require some new planks (approx 4 each side of the keel. The centre spine is also rotten at the stern. I am debating whether or not to take on this boat for a project. I ponder if the work is not worth while since the centre spine is rotten? If it is worth attempting to repair can you help me locate some plans.
Thanks.
whiteleyfamily@googlemail.com