That is a Lycoming 65 HP motor. My Luscombe has a Continental 65. They are very similar. It will definitely windmill as long as the failure isn't catastrophic like a rod punched through the case of a bent valve. With no smoke, oil on the cowl, vibration or immediate prop stoppage it argues for a fuel starvation, including the movement of the mixture to cut off. ?
One of those eternal hangar debates is whether you will get more glide over the ground by slowing to stop the prop to reduce drag but spend time less at than best glide speed, or go straight to best glide and let the prop windmill in a higher drag configuration. I'd suppose there isn't much debate about that in helo world. ?