If the buyer did not do his due diligence as you say, there is no way you have any responsibility in that. As has been said many times here, as long as you were not deceptive..........f*ck him. These are 50+ year old vehicles. The only way any kind of warranty would be in place is if it were specified.

I would not continue to communicate with the buyer on anything related to his buyer's remorse. It may be premature to lawyer up mostly due to the costs. The ball is in his court. There is not much you can do now to change the course of events. Unless you feel the need to buy him off.