Sir Alex Ferguson and Walter Smith sent messages to the squad at a final training session before flying out to Mexico City for the tournament at the weekend.
Scotland won the event in Paris last year and also won it in Denmark in 2007.
At this year's tournament they face Lithuania, Indonesia, Greece, India and Peru in the group stage.
Sir Alex said: "I really hope the Scottish Homeless World Cup team can go to Mexico and do what no team has ever done before: win the trophy for a third time.
"I'm sure the determination, desire and passion the team will take with them will put them in a really strong position to win again."
The squad, managed by social enterprise Street Soccer Scotland (SSS), had their final coaching session with former Scotland goalkeepers Alan Rough and Andy Goram.
Both were in the Scotland squad for the Mexico World Cup in 1986, which was managed by Sir Alex and coached by Walter Smith.
Smith said: "Scotland's Homeless World Cup team did the nation proud in Paris last year and I am sure they will make us proud again this year. It's a big challenge but I have no doubt the team is up for it."
SSS is a non-profit social enterprise that sets up football-related services for socially disadvantaged adults and young people.