Rockers operated by hydraulic lifters are always under slight pressure unlike mechanical lifters that are set with a clearance. That piece of plastic stopped the chain from turning the cam but didn't stop the pistons from going up and down. I'm not saying all the valves are bent. Only the ones that were open when the cam stopped turning are bent.
Before you spend too much money you might go to the local parts supply and pick up a can of valve straightener and pour it in the oil. There are several brands. One is as good as another. I don't think the dealer carries it' They always want to sell you a valve job.
