So did Michael Corleone ever get revenge against those who killed his wife?
There is a deleted scene in The Godfather Part II where Fabrizzio is found...and his involvement with the Barzini family.
...and then another where he is killed.
The Barzini's were, of course, disposed of in that long murder/revenge/power grab sequence in The Godfather.
The source novel makes this clearer.
Don Corleone nodded. “Revenge is a dish that tastes best when it is cold,” he said. “I would not have made that peace but that I knew you would never come home alive otherwise. I’m surprised, though, that Barzini still made a last try at you. Maybe it was arranged before the peace talk and he couldn’t stop it. Are you sure they were not after Don Tommasino?”
Michael said, “That’s the way it was supposed to look. And it would have been perfect, even you would never have suspected. Except that I came out alive. I saw Fabrizzio going through the gate, running away. And of course I’ve checked it all out since I’ve been back.”
“Have they found that shepherd?” the Don asked.
“I found him,” Michael said. “I found him a year ago. He’s got his own little pizza place up in Buffalo. New name, phony passport and identification. He’s doing very well is Fabrizzio the shepherd.”
The gunman said, “Fabrizzio, Michael Corleone sends you his regards.” He extended the gun so that it was only a few inches from the counterman’s skull and pulled the trigger.
Mario Puzo - The Godfather