lawn mower struggles to start and won't stay running.?

My uncle called me over today to help fix his lawn mower. I came over and checked for spark, has good spark, took the carburetor off and cleaned it real good. still won't start. so we took off the valve covers and a broken pushrod and a bent pushrod, and another is not even there. don't know what or why one of the pushrods was missing.

So I went and bought some new pushrods. we took off the head to make sure nothing got inside the cylinder even thou I didn't see how it would.

We put the head back on put the new pushrods in. and went to start it up and it sounds as if it has a open cylinder now. but it starts runs really ruff for about 10 seconds and dies again.

I'm stumped, maybe I missed something putting the engine back together I don't know.

what could make it act like that. and of all things how in the world could he of bent a pushrod in a lawnmower.

its a 24hp twin briggs on a Husqevarna. sorry if thats misspelled.