I have the same problem and just today was going to put in a hardened bolt but may not based on what you've experienced. I did replace the roll pin once with a new one from Lowes and it didn't last. It failed just like the one that came in the tractor when I bought it. 

This is what Agco has listed for the pin p\n 933221     Pin, 1/4 x 2 Spring
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   I did look at my parts tractor and it has the same pin, no surprise it's half broke off. 

 I've seen for the pulling garden tractors that they have available special hardened roll pins, instead of it looking like a c when looking at one end it looks like snake curled up. So the pin is much thicker within it's own diameter. 

  I'm now going to buy the pin from Agco but if anyone else has a fix for this let us know.

Thanks
Eric