Is it worth restoring, well that has to be an opinion, and in my opinion any old bike is worth restoring. People take on bikes found in hedges and ditches and restore them to new condition. My present restoration started of as only half a bike, so I have had to source parts for a start. If you want to do it to make money, then forget it as it has to be a labour of love. As for the registration, if you know what it was it may be possible to claim it, if it's not been sold by the dvla, otherwise you apply for an age related number, but the machine has to be in running order. To obtain an age related number you will require a dating certificate which can be supplied by owners clubs or a chap name Mike Worthington Williams, who charges £30. All the best with it.