How can I realign the drive belt on my Craftsmen lawnmower (model 917-379.1?

The belt which drives the wheels has slipped off the pulley on the engine shaft. I can turn the belt, but can't get it back onto the pulley. Because this has, in effect, lengthened the belt, it does not drive the wheels as designed. How can I get the belt back on the pulley without removing the engine? Also...if the belt were to break, how can it be replaced without removing the engine?