I have an 02 2.5 forester engine in my 97. the 2.2 in it crank walked leaving the bottom end useless. I swapped over my heads to the 2.5 motor and used the 2.5 motor head gaskets being the bore for the 2.2 is slightly smaller.
I used the stock SOHC harness, manifold, exhaust (same heads it came with), etc. The only thing I changed was the bottom end, and the head gaskets. Everything else can be swapped over. I do run premium now as it is higher compression. supposedly like 10.9:1? Its faster than a friends RS and my car is slightly heavier.
main thing is make sure the head gaskets are the same bore.