I wouldn't recommend anything from the hardware store. The metal in those nuts and washers seems to be soft, as compared to what comes on a glow engine. You need a prop washer. The prop washer is designed to distribute the force of the nut evenly throughout the hub, so as not to weaken the hub, or have your nut/washer assembly burn into the hub. I'd find out the size of the shaft and the threads (easy to do), and then order a nut and washer from tower or similar (or LHS). If all else fails, call the manufacturer.