You will be lucky if you can find any parts for the Jacobson. I had a 36 inch walk behind (Commercial) that I gave to a neighbor about 3 years ago. Recently, he went looking for blades for the mower and could not find any in Birmingham Al. Everyone just laughed when he mentioned Jacobson, and said no parts were available any more. Like others have said, you might find a place like Motion Ind that can measure the belt and replace.
I am having trouble finding parts for my 48" Yazoo walk behind (Commercial). It is about 15 years old, and I think it is going to last longer than me. /forums/images/graemlins/frown.gif
Good luck