03 legacy L engine upgrade Q

So my 03 legacy L sedan has over 200,000 miles on it and that fact has me considering engine upgrades. Before i can start my engine research and shopping i need to be sure what engines are compatible with my transmission if any. I done a search for these particulars to no avail. Excuse me if i missed something and give me the link. The engine im hoping will work is a 97 impreza wrx engine i found with pretty low mileage. Thanks in advance. Zach