what to check

When I adjust the valves to the middle of the spec, they are pretty quiet. If they get out of tolerance toward the tight side, they stay quiet. So quietness is not an indicator for tight valves (which is a common way they go out of spec.).

I agree with you lou. I think seller must be use to or thinking of hydraulic lifters & not KZ design.

In pushrod engines with cast iron heads, the valve train wears, creating increased valve lash and noise. (pre-1970s cars, early HD and other pre-war agricultural implements.)
In KZ engines the aluminum valve seat is the soft spot in the system, so lash and noise decrease with wear.

That wobble can be caused by bad tires, worn or improperly adjusted steering stem bearings or swingarm bearings. If the tires are more than five years old or are showing signs of cracking, they should be replaced.

How about front wheel bearings, also a cause? Do you recommend All Balls bearings as quality?

That'a what I used front to rear, top to bottom.
Steve