Ugarte is not a "a traditional No. 6" - you're just demonstrating that you don't really understand what you're talking about and what the problem is.
Ugarte does not sit deep and shield the defence, he's a player who only really works by proactively hunting the ball and chasing it down. He's as much like a non-goalscoring Conor Gallagher.
Similarly, Tonali is also someone who prefers making tackles on the front-foot by pressing, and is not so good at defending on the back foot and protecting the back four, or halting counter-attacks. I'll remind you again of a comment from the Newcastle forum I posted on here earlier: ""Shite. Bombs forward, loses ball and leaves us woefully exposed". That is not what you look to your 6 to do; they need to be more disciplined and focused on progressing the ball via passing, while they generally maintain their position on the pitch.
That's not the only reason why Casemiro is hard to replace; it's not even the main reason - he is excellent at defending the penalty box, whether by tackles, blocks or headers. Ugarte is not. Casemiro is also excellent at passing forwards from deep. The goal-scoring from him has been a very welcome bonus, but it's not the first thought when looking for a player in his position.
A player who is excellent at defending the box and at passing forwards/creating from deep would probably suffice for a Casemiro replacement, even if you ignored his play in the final third. But even then such players are unbelievably rare at the moment.