As Julian said, the drive flange is bolted to the brake drum, so this is a good place to start. Bolts tight, tab washer strips in place. Replacement drive flanges are available, and a new one will reduce the slack in the drive. All depends on the amount of wear on the splined wheel hub, which unless you can find a better example is what you are stuck with. A new hub washer rubber 67 6051 will also damp out the clunk, most are oil soaked spongy excuses for the real thing, like on my bike.
Swarfy.
Additional. Those rivets..... I have seen hubs with nuts and bolts in place of rivets, so either a common backyard fix way back when or a dealer mod. With a bare hub the rivet holes can be plug welded to the centre tube. The join of the two outer crinkles can also be V'eed out and another bead of weld added to give a good strong bond to the three components. It's a p** poor design in the first place, a lot of changing loads on a small area (but good enough for the cost accountants).
