I worked as a bike mechanic in the 1970s. At that time I had Suntour and Regina clusters on my bikes. The Suntour was easy to change sprockets, the Regina a bit more difficult. But if you pick up some used ones, you can mix and match sprockets, etc. The Suntour ones of that era could develop a clunk, I am not sure but I suspect that the bearings were not shimmed quite right, several of us mechanics at that bike shop had Suntours with a slight clunk sound when pedaling hard.

To change sprockets, some really good big chain whips were needed. Anything from that era might also need some penatrating oil if you tried to change sprockets.

I changed some of the sprockets on both of mine to get custom gearing, so that is how I know that the sprockets can be changed.