The Murrays use very unique belts that have to the exact size and pitch. Nearly impossible to improvise belts on those.
First, make sure it is the correct Murray belt.
Next, inspect deck linkages for being bent. Make sure a spring to pull the deck forward isn't missing. While inspecting, look for adjusters for the travel forward(blades off).
Last, sometimes there are extra holes in the brackets that hold the deck on the mower. If the deck has been removed recently, make sure you used the right holes. Could be a rusts pulley causing extra friction. If it smokes a lot, it sounds like the belt is routed wrong.