As an under 40 (just) latecomer to Speedway I never saw the old 2 valves, nor indeed the upright engines. I have wondered though whether the narrow band of power put out by a laydown engine really is suitable for some of the smaller, narrower tracks in the UK. I'd have assumed at around a track like Wolves used to be, or say Edinburgh, then an upright engine with a better balance of throttle control would work better.
I've noticed that a lot of riders tend to ride a diamond shape on a track as opposed to an oval, narrow into the corner and clattering the fence mid straight to enter the corner tight again, is that down to the engine or just the rider style? I've seen so many riders hanging on for grim death that you wonder if that's really not a terrible accident waiting to happen.