^Your new 2.2 will be an interference engine.
Joaks, like the guys said above you will have to pull your engine to replace the oil separator plate. If the engine is leaking back there it could also be your rear main crankshaft seal, which will also require you to pull the engine or trans.
Do you have a 5MT or a 4EAT?
The good thing about pulling your engine is you can replace everything else why you are at it. Also, like the guys said above it's not too bad to pull a subie engine. It's just a bit daunting the first time but afterwards you will know your way around a boxer.
Are you sure that is where the oil is coming from? You can get U.V dye that you dump into your oil, drive it for a bit, then shine a black light on it and see where the oil is coming from. I would power spray the bottom of the engine first. You can pay a mechanic to do this. It's not too.
What other seals have you replaced? Common sources of leaks are valve cover gaskets, cam seals, the crankshaft seal, cam cap 0-rings (aka camshaft support o-rings), oil separator plate, and the rear main seal. Check your power steering pump as well for leaks. It can look like an oil leak.