Under a modern-day perspective, a crusade is any undertaking done with a strong believe in the cause. There are no limits to a crusade: The mission and the vision that are involved in it are strong enough to meet the goal. It is basically an ongoing combination of strategies and plans that make everything take a specific turn for the best.
While Willy Loman does have a mission and a vision in life, these are not necessarily the ones he should be working towards. Willy simply wants to be well-liked, popular, and aggressive in order to become a powerful salesman like his model, Dave Singleman. However, Willy's formula limits itself to imitation. There is nothing unique or different in Willy's quest for the American Dream. If Willy were a true fighter and a true creator of his destiny, he may succeed at getting what he wants. However, we know that Willy only lives for the moment and his magical moment does not seem to ever come.
Yet, we could can also agree in that Willy's tenacity, obstinacy, stubbornness and resistance to change are all symptoms of his extreme desire to succeed: He does have a hunger for success. Being a successful salesman is the Holy Grail that he so wildly seeks. The fight to find it is his own personal crusade.
Therefore, Willy's strong belief in his personal dream is what makes him so complex, headstrong, and determined to become what he wants to become. The reason why never becomes it, is because believing is simply not enough: A conquest, like in the crusades, can only succeed after a good strategy has been put in place and has been carried on. Willy has no strategy, but he still has a huge penchant for dreaming. Therefore, his dream is ultimately what allows him to survive his personal crusade.