Well, I've found some answers. Not sure how I missed some of this the first time- Guess I should have spent more time investigating and set up proper lighting. Anyway, these pictures should tell the story:


The crank key was broken when it came out, and more importantly, had worn thin! Until I took the key out, the rust had hidden the fact that the crank slot was significantly wider than the protruding portion of the key.



The snout has unfortunately taken a beating. It is no longer smooth and round, and what I had originally thought were only rust patterns are actually low spots on the crank.



The areas of wear can be seen on the sprocket.

All this adds up to a loose fitting sprocket that can play enough to wobble, and may not be aligned when tightened down.

It appears that this thread describes another incidence of this problem. That user (mk75) apparently ended up purchasing a new long block. I would like to hope that mine is still salvageable. I have a new sprocket on the way and will get a key as well. Any ideas for filling the low spots and attempting to get a round crank out of all this? Part of me is tempted to fill the splines on the sprocket so that it will ride on the high spots left on the crank.

The "redneck option" of JB weld has crossed my mind, but even if I that stops it from moving, there's no guarantee it will end up properly aligned when tightened.