So to hit some of the excellent points addressed-
The woodruff slot in the crankshaft seemed to be in good shape when I last looked at it, with the key firmly in place. The keyway in the crank sprocket is where the slop comes from, and even though the sprocket appears undamaged, there is noticeable play. Odd. Sounds like I shouldn't have too much trouble getting that key out, then I can sort out the proper replacement. The crank sprocket itself will just have to be replaced.
As for the crankshaft bolt, yep, it's the pink one, all clear on that front.
To clarify a bit on the crank/key/pulley relationship, check these shots out of a pulley and sprocket:
Sprocket Pulley The only thing aligning the pulley and sprocket is the roll pin, with centering coming from the sprocket OD and pulley ID. Through hole in the pulley is for the bolt only, no part of the crank passes through. My apologies if I am mistaking anyone's comments, but I wasn't sure if all the Monty engines used this configuration or not. From a design perspective it seems odd to me that the crank doesn't extend all the way through and key into the pulley.
So with reference to Dan's comments, with this setup I've read of instances where that roll pin shears and lets the pulley spin around the bolt/sprocket, while the sprocket remains keyed to the crank and thankfully keeps those cams turning. In my case the pin is intact, but it would seem that the root of my wobble would be the slop in the sprocket... Any slight play at that sprocket would only become worse out at the end of the pulley. Not sure I can get a decent look at the back-of-pulley/front-of-sprocket interface while it's running with the t-belt cover currently installed.
Still hoping it's the bolt that's too long, which would be the simpler of the fixes!
