The main problem would seem to be the Lay Down Engines. Speedway is all about sliding a Bike - to do that properly you need Dirt. You cannot give these Lay Down Engines Dirt as you explain in your excellent Post below. The Sport used to be known as Dirt Track Racing and was a great spectacle - today however - it is still a spectacle but, without Dirt, it simply isn't the same or as safe. That is though, only my opinion. 
  
 
Great Post. 
  
I am prepared to admit that you know more about the Technical Side of Speedway than me - so I accept what you say.