Hey, the Junkion mold includes two posts in the robot mode's butt that snap into two slots in the seat of the motorcycle! Also pegs and holds in the sides of the bike and legs of the robot that don't quite line up.
Junkheap does have a new head after all. It's too bad, really--I doubt anyone will buy him except collectors, and every collector will display Wreck-Gar sitting on him, so no one will ever notice the head.
If toys had feelings, that would be incredibly sad. But still true, nonetheless.