Canada has four stadiums that meet (or will) FIFA specs Commonwealth, Olympic, BC Place, & BMO* (upgrading to be over 40K.) Mexico has six: Azteca, Olímpico Universitario, Jalisco, BBVA, Cuauhtémoc, & Akron. Five of these are already selected as WC'26 sites. If the US is dropped, they can step up.
Mexico will get the opening ceremony, one semi, and final. Canada can maybe get an extra group and and even split of the earlier elimination games? I think only Olympic in Montreal meets the 60k seating for a semi, but in BC Place & Commonwealth are close (FIFA could waive, given the circumstances)