Yes, string will work. You want to get a 'V' shaped belt. You can get them at auto parts stores or Tractor Supply. If you go this route buy the belt you think you need and the size above it and the size below it - trust me. You can return the ones you don't use. Another thing you can use is 1/2 inch clear tubing and a connector from the plumbing section of Lowes or Tractor Supply. I had a really old table and a newer clutch motor. I had a very difficult time finding a 'V' belt to fit. My old Sewing Machine Guy told me about the tubing. He said this was for emergency or light use only until I could get a proper belt. I used the tubing set up for quite awhile with no problems but I did keep a watch on the connection. You can also get on Ebay and order a 3/8 inch leather industrial sewing machine belt. It will come with a wire connector and instructions for installation - this is my preferred method.
I hope this helps you out. - Tracy