It looks to me like the crank pin had a crack that had been there for some time.
If you look at the last pic, it looks (to me anyway ) like a crack had existed for some time. Look at how the crank pin broke off flush with the face of the throw.
From the 9 o' clock to the 2 o' clock (almost half way around ) it looks like there is rust in / on the fracture line.

Looks to me like plain 'ole parts failure...rust might have been a contributing factor, but not the cause. If the engine had been run lean, then detonation might have caused the crack / breakage? The glow plug element looks frosted...that could be another clue?

The cam bearings are rather rusty, yes, but the rear main bearing and crank counterweight don't look all that bad. (I've seen MUCH worse )

gklept...does the rear bearing still rotate freely or is it rough or loose?