The belt is actually too wide for the back motor pulley, so it's not fitting in the slot. I have one other older Singer belt. I'm going to try that. It's a bit cracked, so it won't be permanently used on the Pfaff.
Don't worry, Leon, I have no intention of letting this machine go! I'm really proud of it and can't wait to use it for my applique projects. I don't give up on machines that easily! If I couldn't use it in the house because of vibration, I get my DH to built me a sewing shed with a cement pad foundation!