I think you'll need to loosen everything up, un-jam it, then proceed with the PDF instructions for leveling the head, as normal.
The thing that concerns me is the broken part you mentioned. Not sure if the head will hold level or not...I sure hope so.
Here's some links you may need:
DOWNLOADABLE PDF INSTRUCTIONS
Checking and Adjusting the Head Pressure (Overall head pressure doc that does not include the leveling but does include the guide rods)
Leveling the Head (head leveling only)
Adjust Vertical Rods (Adjusting the vertical guide rods only)