As I remember my 42C deck, there are several belt retainers which would normally keep the belt in place. Obviously, the big one around the front motor pulley, but also at least a couple that are mounted right beside pulleys on the top of the deck. If you have a deck parts diagram, it should show each of those. If those are all in place, next thought would be a missing spring so not enough tension is being applied. I have both a 42C and a Freedom deck and sometimes mix up just how each is rigged, but I believe there is a tension spring on the 42C.

Look for the idler bracket where it is welded to the deck. The welds get pulled through the deck and then you can't keep tension. Another issue with these is the PTO stalk can get floppy and loose.