I think that you are saying that the "shaft" that the rocker arms move on is broken.
This shaft is under the metal cover at the top of the engine & lies along the line of the engine. That is to say, front to rear.
Is that correct?

These rocker arms push the valves down to allow the gasses into,... & out of,... the cylinders.

If this shaft is broken, it will be necessary to find the reason why. Lets go one step at a time & make sure that we know what it is that is actually broken.