The SR is not just Japanese domestic. Sold all around the world, but not US.
Dazza knows more about this than me, but the differences include road legal electrics with NO battery and plenty of alternator output, the oil injection system, steel fuel tank, heavier flywheel (nice), 28mm PE carby (restrictive, but torquey and economical), flange-mounted exhaust, 19inch rear wheel, slightly wider ratio gearbox (nice) and apparently in some countries, a different head.
It's easy to convert the SR's to pseudo-E-model specs by adding a 35mm carby and plastic fuel tank, etc. The engine porting is the same and with the flywheel and gearbox they make superb trail bikes.
My bike and Dazza's have had the injection system removed, maybe just to eliminate another variable. I kind-of wouldn't mind to have it back to avoid the premix hassle. As for hooking it up to a non-injection KDX, it would be possible, but you'd need to be keen. It would involve an engine case and the pump itself, plus an oil tank. Maybe more.