2 Valve/4 valve, Upright/Laydown the difference is the progress that Riders/tuners want & it appears they want an engine that revs way more than it is required to do so. many things have changed over the years in speedway & we all look back to say how great it was way back when.
Todays engine as a a laydown is much the same as it was 40 years ago. The insides have changed, to achieve the massive amount of revs todays engine make they have moved to a short stroke engine which makes for smaller flywheels & bigger pistons to get towards 20,000rpm & then fit a limiter to reduce it to 13,500rpm. The only time an engine might go over this is on the start line when the engine is not under load.
The quickest solution would be to limit the bore size this would increase the stroke to make 500cc & dramaticly reduce the upper rev limit also reducing potential noise levels. With lower revs it would also reduce maintenance costs for the rider. This would also increase the useable rev band making the machine more rider friendly for those that choose throttle control over twist"n"go