I used to build those. The seat was a bit too tall for many of my customers, bit otherwise it was a good build. Had no problems with the rag joint on them.
When centering the sprocket, put all 9 bolts in loosely then tighten every third bolt until just snug, then use an old drill bit or other piece of steel to put between two teeth to knock high spot down with a light hammer until it is centered nicely, then continue with tightening all the bolts to get wobble out while periodically checking that center is still correct.