As far as i know, you would have to buy the logic board.
The only reason i say this is because the SIM tray is attached to the board permanently. and the ejection mechanism is built into the tray. That being said, one possibility is to go through the disassemble steps outlined here all the way to step #13, which should get you in good eyesight of the SIM tray, and its faulty ejector.
From there, you just need to get your DIY geekery on and see if you can wedge the ejector back in front of the gold bracket that pushes the SIM tray out. but be aware of poking in the wrong places, and static electricty. One little zap and your logic board is barbeque.