Ditto to Alainve, the EJ20 (came in both the Liberty and Impreza) are a very wide engine. Even in the Liberty which is a wide car compared to a TA series, its still a nightmare to get the spark plugs out past the chassis rails! It would require MAJOR work to get it to fit.
You might have batter luck with the turbo 1800 boxer motor out of a L series Subie, but dont quote me on that!
Also, Subie gear boxes are weak if you want to drive hard. Even with STI close ratio internals we managed to destroy 2 boxes last year rallying a Liberty. 2nd and 3rd gears totaly destroyed! somthing to keep in mind.