It's sort of not that bad, unless that rocker is actually missing. The shaft doesn't have to transfer torque, it just holds the rocker pivots, so as long as it's not bent (wouldn't take long) you're just starving on 2, and 1 if the last bolt sheared while running. Pretty bad then, but a testament to the strength of these old engines that it ran at all. As long as there's something left of the rockers it'll let some spent gas out and not blow itself up.
Surprised there isn't more damage if rockers and spacers started falling out.