[n4mwd]

When you overtorque the rocker nuts it snaps the little keeper feet off. This sets up a chain reaction. First that one wiggles loose. Then it works its way under the chain and sprocket causing the sprocket to hop a link. When that happens, the pistons impact the valves (The Ninja is an interference engine). When the valves are impacted, it breaks off the rest of the rockers. The fact that you had a few survivors amazes me. You need to remove them and check them over for any signs of bending.

Also, as Motofool said, you need to inspect the valves and pistons for signs of impact damage. You might be able to get away with one of those inspection cameras that can fit down the spark plug hole, but sadly, its best to pull the head and replace the $25 gasket when you put it back.