You obviously have to give to add a prime age established scoring winger to your team.
Hage and a strong 1st round pick (top 2 protected) would be the main value.
However the Blues have a glut of forward prospects on the cusp: Dvorsky, Snuggerud, Stenberg and Stancl (who is making a big move right now) all look legit and in similar timing as Hage.
[Major RHD prospect is also a Blues need but not too much else ... you offering Reinbacher?]
Montreal has plenty of wingers and shouldn't be looking at guys like Buchnevich. The premise of this trade makes no sense apart of the unnecessary idea of getting a Russian in Montreal.
A Russian addition in Montreal would need to be a top6 C or a top4 RD.