@audiometrix_hearing has been selling the Classic/Serial Hi-Pro along with the USB cable in a bundle. Here’s a link; That’s not the actual cable price (I just made it up to include individual prices in the bundle). He may also sell the USB to Serial cable separately?
Though, I really don’t know if this will solve your problem. Maybe @Volusiano could say if this USB-cable/Classic with Serial Hi-Pro combination has worked on Opn Firmware updates?