Can I swap Engines between SX/SS175 & SX/SS/SST250?

Yes. Engines are basically the same, only the Bore is different. Engine mounts are the same on all SX/SS/SST 175 & 250 frames. Carb must be replaced also (27mm / 32 mm). Rear sprockets differ on 175ccm and 250ccm bikes and it should be replaced as well.