I purchased an upper pulley from someone on the forums. He got the pulley from paramount performance. Will I be able to use the factory belt or do I need to order a new one? I asked him but he wasn’t sure.

 

I'm 99% sure you don't need a new belt otherwise all the various sellers of upper pulleys would also include a new belt as part of the deal and I have never seen an upper pulley advertised this way.
The size difference is just too small to require a shorter belt.

 

+1, 100% sure.