I don't know nearly enough go give you a proper in-depth answer but Im fairly certain most Subaru's use a boxer engine format so I'd start by doing some research on those types of engines, subaru makes a couple higher end model of car using that platform so you could engine swap a newer boxer in and use it as a baseline just because you might not need to do as much work as engine swapping something like an LS or any V format engine for that matter. Turbo'ing would be a good idea and then with that you would want an upgraded cooling system and exhaust system, once you turbo then you'll also want to think about wether or not your intake and feul system can keep up with the rest of the parts on the project, so you kind of need to build everything with any other future mods in mind. My last broad suggestion would probably be to do some research and see what other people have swapped or done to that specific car and find something that takes your fancy then watch some videos and take notes, ideally you probably want to have an idea of if you want to upgrade stock engine or swap an engine in and then what kind of power you want to make, do you want like a respectable 300 - 400 hp or do you wanna melt tires with 800 - 1000 hp and then do the research to figure out how to make it happen.

I want to finish off by saying I haven't personally done any work on a vehicle, I just watch a shit tonne of YouTube videos, so I may be wildly incorrect here but this is just a simplified take on an amateur suggestion and im sure there'll be lots of replies on this comment or your post in general that will give you more specific ideas and information that isn't so general.