At best, we live in a world that has moved away from what was classically the norm. Today, I am just as screwed(?) up as most over fastenings which are fitted to my machinery. You are far more fortunate that you can change the drawings. Most are not written in tablets of stone and unless you are an utter stickler, you can go to metric which is the coarser thread system.
Frankly, I would go for what is still available in the Imperial System ie the 40tpi. If the old BA stuff is not available, you are stuck with metric replacements.
It is worth looking at what George Thomas had to say in Model Engineers Workshop Manual about 'the changes' True, I am a bit biassed towards him but you will glean the possible replacement sizes which he gives.
Digressing further! Like many people who are motivated by the joy of model engineering rather than greed, I was instrumental in publishing old articles from Model Engineer. I had the set of Martin Cleeves articles on Making Socket Headed Screws to post.
Of course, Cleeve is dead or should I say, Hart, but Magicalia who now own ME and MEW have demanded that all old articles be removed from the net. In another site, the present editor is trying to find the next of kin. to him and many others.
As a ME and MEW, contributor of the past, I'm a bit like John Inman in 'Are You being served'
I'm Free!
Norm