The blades won't engage. I have just replaced the serpentine belt and that didn't fix the problem.

Hi,
Just a bit of reality here, but I doubt the movement of the engagement arm/idler to perform the task. At any rate I can give a link to your manual:
http://www.poulanpro.com/ddoc/PPOO/P..._583939424.pdf
And page 17 has the deck and its belt.
Peace,
Clarke