usual problems with jumping gear in a constant mesh gearbox is worn dogs due to changing gears with throttle (without disengaging clutch or badly adjusted clutch cable), or worn gear change forks, due to riding with your foot on a gear lever. it's a general diagnosis.Sopwith Camel wrote:Do you know this bike or have seen it?
those rotax engines are really tough, so the failure is probably due to negligence. I would just swap the motor, with other from ebay.