If it's not too bad you can sand the rubber pulley down by spinning the motor and holding a sanding paper next to it, at least as a temporary solution. It evens out the bump, takes of the hardened surface and it grips better again. The pulleys are available online at least;
Sew Classic, ebay, perhaps your local repair shop have them. With an external motor like the 43 it's an easy replacement. It's all about identifying the correct fit or tweaking one for it to work. I hope you soon get it out of the barn and into your home ;- )