There will be a missing bolt hole on the motor for the second starter bolt.
Early tranmissions had stud on the trans and no hole on the motor.
Later transmissions used a long bolt through the bellhousing and a matching threaded hole on the block. So some fabrication will be involved....a search should turn up what others have done.
Not quite sure of your plan, but USDM WRX wiring and ECU will not control an early EJ20G. You should be able swap the cylinder heads and intake manifold and the crank sensor to the older motor. Basically just use the shortblock, but double check compression ratios and head gasket thickness to be sure.