To my understanding the S/R had the same engine (with addition of the blue valve cover). It has a different ECU, intake and exhaust. The ZX2 and focus engines had different heads, cams and the ZX2 has VCT. I think the block and pistons are the same.
My Ride: 2007 Mazda 3 2.3 Grand Touring Sedan MTX
Wife's Ride: 2003 ZX5 MTX [SONY Heatunit][ST Knob][SVT Pedals][SVT Mirrors][SVT Silver Handles][K&N Filter w/ SVT Box and Snorkel][NGK Wires]