The engine will mount up exactly the same but you will need to use the flywheel and clutch off the non STi engine (unless you also use the STi transmission)

The wiring in the WRX will not support the active valve control that the STi engine has, the ECU will not operate it either. Changing the entire wiring loom and ECU is one solution. Adding the extra control wiring is an option but is definitely not a DIY.