I used a modern ‘X Ring’ chain from eBay, for peace of mind really. It has bling gold sideplates, but no one has bugged me about it yet. I found it with a simple search, it had CB750 K1 in the list of bikes it would fit.
The downside was it came only with a rivet link, no split link, there’s a thread somewhere where I asked about a riveting tool, I got one from Germany via the Bay, rather than take any chances. Absolutely no problems since, and it only needs occasional attention.