This is an old mower which most likely has significant wear on the pulleys.
Thus it might need new pulleys or some times you can cheat by using a smaller belt.
Also check that all of the belt retainers are still in place
The ones around the engine pulley are often missing on old mowers.
Get a parts manual and check that all of the listed retainers / guides are still there and close enough to prevent the belt from jumping off the pulley.
I am not familiar with this particular mower but if it has a moving pulley to tighten the belt to make the blades spin, check the pulley and in particular the arm that the pulley sits on.
The holes in them where they pivot flog out oval over time which allows the pully to either go looser than it should or sit a a funny angle or both.
And either of them will cause the belt to jump off .
IF it happens all the time , get some one to sit on the mower and you watch the belt from the non discharge side to see where it jumps from
I have a Go-Pro ( cheap copy ) camera than I clamp on to the foot board of problem mowers so I can watch the belt when mowing if all of the usual suspects appear to be "not guilty sir"