Could be many things including just off tune. Make sure no pin holes in fuel line. Are there any bubbles in line going to the engine and I assume the pressure line is connected.

Is there any mechanical damage to the engine, was anything bent? Even if you think you straightened it out please list ANY damage found. Don't bother with the broken prop, that is assumed.

Sorry for the loss of the plane but I haven't seen one of these fail this way before and there have been several thru my field. Did an experienced flyer look over the build prior to flight or check it after the accident and see if more could be determined? Alpha's normally perform quite well.