The tool solution in reply #145 above got me thinking about how to tighten and loosen the new sprocket without the engine together, or just more elegantly than the usual chain-wheel-brake-2x4 method.
The sprocket has three holes. I don't know what their purpose is, but I can make use of them.
So I designed an easy-to-make tool. A piece if steel bar, drilled for a couple of bolts. One bolt fits in the larger hole (deeper than shown here) and the other rests against the outer edge of the sprocket or on top of the chain if it's in place. The question is whether there's clearance to get at the nut. Gotta go measure. Maybe the tool attaches to the back of the sprocket.
It would give me the leverage I need to keep the sprocket from spinning.
I think the same basic idea would work on the pulley. One bolt goes inside the lip, the other outside, against the belt. Or turn around the bolt further from the end so the head sticks out and engages between the pulley teeth to keep the pulley from spinning between the bolts.