There probably was a coverup but I think whatever crashed was probably either an experimental craft that the US made and didn't want the Soviets to know about, or a Soviet probe we didn't want everyone to know about. Think about it, how much more likely is Stalin than Xenu to be behind this?
My favorite UFO hoax is still Orson Welles Mercury Theater War of the Worlds scare, even if the effects of it were greatly exaggerated.