I don' t know what you are trying to drive but back on the farm many things were belt driven. To reverse we would simply flip the belt between the drive and the driven pully. Another way to say that is that the belt would come off the top of the drive [motor pulley] down to the bottom of the driven pulley around the driven pulley exiting from the top and back to the driver entering the bottom of the driver. if you trace this out with your finger you will see that the spin is reversed. you can do the same with two gears set side by side the drive gear will produce opposite spin to the driven gear. the right gears require less attention than the belt drive but the belt drive is faster to set up. it can be tough on the belt because of the friction at the cross. another option is an idler pully riding on the outside of the belt. I hope I haven't wasted your time.