A 1950's 200 cc bike is an unknown quantity and who is to say that Villiers engine hasn't been breathed on and that Greeves isn't closer to their race bikes than road bikes.
While I sympathise with the lad, if he wants a licence, then he needs to pass the test on a 125. I wouldn't want to see what we had in the 70s when I started which was buy a 250, jump on it with zero tuition and head off. FYI it was the Honda dealer here in Edinburgh, Graham Chapman, who lobbied for the 125 cc rules and CBT in the 80s to replace the 250cc free for all. He said he was getting a guilty conscience about the number of 6 month old crashed bikes he was recovering from greiving parents.