Since the government of Turkey is claiming that someone within the US is a threat to their national security, a leader in efforts to overthrow the Turkish government, and an all-around bad guy …
… and since they have requested extradition of this person to Turkey, and the US has refused without more evidence …
… would Turkey be justified, based on US actions in the past, in either blowing up the person with a drone strike, or sending in a covert assault team to kidnap him?
It's hard for me to think of a reason why they would not be, again, based on what the US has done with drone strikes and extrajudicial kidnappings. And that's precisely why such things extremely dangerous for us to do.
I doubt Turkey will actually do such a thing, but more because of fear of retaliation rather than concern over international law. But that just reinforces the idea that "might makes right," a principle that the US is loath to state as public policy.
Would Turkey be justified in kidnapping or drone-killing the Turkish cleric in Pennsylvania?
American Exceptionalism in its purest embodiment: the U.S. is not subject to the same rules and laws as other nations.
Justified, not necessarily. Following precedent, absolutely. Though by now we should all be aware that the US plays by a different set of rules. It expects extradition on its behalf but rarely grants it.