The legal option
Only consider legal action after all other options have been exhausted. Do not be vindictive; be clear about your aim. Do you want the repairs to be completed, for example, or do you just want your money back? You will need to prove your argument, of why the work is sub-standard, in consideration of the sum paid. Gather second opinions about the restoration work from your car club, other traders, or an independent classic car condition report to support your case. Usefully, taking out legal expenses cover with your home insurance policy can help to cover not only the cost of an expert opinion of the work, but also legal fees.
Going down the legal route really should be a last resort – and you need to be well prepared.