drive chain

Exactly right. #50 is the right size, but isn't durable. You need 530 chain, for high speed, motorcycle use. Most older British bikes can't use the heavy side plate or o-ring chains, as they cut into the crankcases, there isn't enough room for them.
I found something which may be interesting to you. My 68 was running as a slightly hotted 750 with five speed and 20T gbx sprocket. I was getting some serious wear on my rear chain, less than I thought it should get. I replaced it several times, same type of chain (Tsubaki QR). Then I replaced the rear drum and sprocket with a 43 tooth one piece item. The wear dropped to maybe 15%, or less, of what I was getting previously. Nothing else was altered. I attribute it to the "hunting tooth" principle, the 43T rear sprocket being a prime number. It certainly isn't explained by the difference in gearing.
It might also explain why the factory chose 47T for the conical hubs and later disc brake bikes, and 53 for the Trident?