Anji,
Hello, and a very big welcome to the forum. It is excellent to now have a member in India. As I recall, we have had one guest from your country ("fbm") but he never joined. It sounds like you have an excellent project going and I (and, I know, others) are anxious to see photos.
I think I am not the best person to answer your technical questions, as mine is a '55 swing arm
(
http://www.a7a10.net/forum/index.php?topic=166.0 ). I trust you will get good advice from several of our members with like or similar models. One thing we have in common is long outdoor storage/abandonment before starting the rebuild. In my case, several years outdors in Los Angeles (yes, it does rain there) under a rotten tarp while the barrels were off. The crankcase was full of emulsified oil, stagnant water and rotted leaves. It must have made interesting chemistry, as the crankshaft was presereved therein (though, it has since been reground).
I am guessing that there were a large number of BSAs of all models shipped to India and that there are still many there and running or waiting to run. Oh, to have access to them!
Bye.
Richard L.