generally piston rack is the first to go.. but it depends on the materials of the components...
a cylinderhead is unlikely to break unless it's plastic and you have a metal piston head.. or vice versa... Then when you have both metal, it could lead to a V2 gearbox shell cracking at the front.
If the piston has a steel tooth rack, then it's likely that the gears or bearings that hold the gears break. But there have been cases of the entire steel tooth insert slipping out of it's position and causing all sorts of mayhem. That's why some guys super glue or epoxy it into place.
If you have an aluminum piston, it's a good chance that the rack doesn't break, but the pickup tooth and the entire back end of the piston tears off.
gear breaking can be teeth coming off, axles bending or shearing completely off.
Bearings can bend and not spin freely or spin off axis. That's why some people use bushings.
The front of the gearbox shell coming off is normally a v2 issue, v3, v6 and v7 aren't as common... if at all. But the potential is still there because of the cylinder window.