You can't choose the piece for your opponent.
If he makes an illegal move, you can stop the clock and find the arbiter to complain. As long as it's a first offense I'd expect you just get some extra time on your clock.
If this happened to me I woudln't want my clock ticking down, so I'd pause the clock and tell him to go get a queen.
If I had a forced mate I woudln't ask him or anything, I'd just play it :)
Hi chess friends
In my last OTB game my opponent played pawn to b1 , left the pawn on the board and pressed the clock.This is obvisously a rule violation,as he has to replace the pawn with a piece. Since I had mate in 3 moves and only 4 minutes left i did not cared about . I played my next move h8Q+ correctly by putting the queen at h8 and asked him what kind of piece is standing on b1. He said a queen but it was still a pawn. He argued that no queen was available, but as I asked him he found a queen which he placed at b1. In two moves I checkmated black by Qd8+ and Qd6 #. But if my h-pawn would be at h6 at he plays the b1 move incorrectly can I claim that he has to take a minor peace instead of a queen because he has made an impossible move b1pawn. In this case winning or losing depends on what piece is standing on b1. I am looking forward to your replies as international chess players. What are you thinking about this.How would you behave in such situations ?