Are you trying to imply Iran wouldn't assassinate the United States President if they could? I'd like to call your attention the most highly trained professional body guards in the history of the world - the United States Secret Service.
However, I can't leave it there, because that would be dodging your point. And your point also brings up a larger issue: escalation. Where does the "you killed my guy so I can kill your guy" stop?
In theory it would be acceptable. Practically, of course, that won't and can't happen. Regarding escalation, however, you could draw a line (make a "rule") that the attack and the retaliation are it - nothing more is allowed. In other words, a one for one policy. I know practically speaking of course that will never work.