I have also done a 4WD conversion. You'll need the transmission, driveshaft, and basically the entire rear suspension.
driveshaft carrier bearing has nowhere to mount. Several ways to do it, I had a one-piece shaft made....
Rear diff hanger will have nowhere to bolt. I drilled and tapped, which worked great for something like 7 years and 60k miles (car was killed last summer when the last owner drove drunk and crashed it.....rear diff still attached).
RWD....depends how you want to do it. Do a 4WD swap with a true 4WD Subaru transmission, lock it in 4WD, disassemble the front axle shafts, beat on it until it breaks. Transmission output will probably be the weak link.
That's about the only way with the stock engine. there are several options for beefier RWD transmission that will bolt to an EJ/EG/EZ/FA/FB Subaru engine, but not for an EA. It is possible to make a custom adapter plate/flywheel/clutch to use something else...but it would be a one-off.