If there was a dead cylinder the engine would have a very rhythmic *thump* every time that cylinder discharged unburned air/fuel, and the entire car would have a rhythmic vibrating pulse. I had a dead cylinder when I first started my rebuilt engine that was due to a clogged injector.

Rough idling and loss of power can be so many things that its impossible to tell with what you've told us. Changing ignition components is a good idea.

You say a spark plug was broken...it is possible that you have a damaged valve or valve seat becuz that little piece got bounced around for a while. Best way to tell that is to have a shop run a leak-down test. They'll pressurize each cylinder and check for excessive air leakage, and they have ways of telling where it's coming from.