It's usually not too hard to figure out a fingering that works. Just don't do anything that feels really bizarre or involves stretching in a weird way. As you get into harder music you have to have an open mind -- it's not unheard of to do weird finger-crossing (like having a scale end with 4-5-4) or playing a line with some of the notes covered by both hands or crossing hands. But for the most part it only takes a few seconds to find a natural way to play any particular passage.