Can anyone help me identify what is broken and where to buy a replacement?

 

Ill ask the obvious question whats the year make and model of this unfortunate atv?

 

Can't tell because the dirt on it, but is there a hole in that lower swivel/cross tree, that the bolt in the middle of the king pin screws into? If so, I would guess all you are missing is a spacer which would go between king pin and lower swivel. Clean it up and take a picture of the other side's king pin assembly, which should show what is missing. Also answer nny's question.

 

​​​​​​Pretty sure this is it. Had on go on me last summer. Available at vmcchineseparts.com

 

Absolutely certain it isn't. That "tab" has the swivel tube built in. On the OPs pic the swivel tube is in the A Arm. If they had cleaned it before the photo you could tell, but I would guess there won't be anything wrong with it anyway, just lost the lower spacer and nut, if it isn't a captive nut.

 

It's the spindle and spindle bracket all your missing is the nut that holds the bolt in place