Great job, ex. Now, I think the Tyson one is perfect:
However, A few things about the Vitali one-great job though. It's just, I feel the jawline could come down a little, the lines in the forehead more pronounced, the brow a little thicker, and also these days, if you're going more current, he's graying, tends to be stubbly and I think his eyes might be blue.
I'm thinking, more like this, just as an example, not a replacement, something to work off of:
Notice, I lowered that jawline, added some subtle dark gray stubble, hair and brow more light gray (it's all about nuance, I feel), thicker brow, blue eyes (shade may be wrong), and deepened the forehead lines. I took facial portrait science at Caxap U, back in 1945 though, so, I'm perhaps, no offense, working on some more "technical" knowledge than most artists.