Add a tag for Scotland as you did for Muir to get more attention.
My way of trying to connect to a certain ancestor is to start by building out all the families named Muir - children, aunts and uncles, cousins in the time period that is easy to access. The more width in the lower branches of the tree the better the chance of finding the higher branches.
It might also help to chat with PMs of John Muirʻs profile and his descendants. Look at Johnʻs descendant list for anyone in Wisconsin.
I am sure others will have other ideas. Donʻt give up.