actually to be fair to him, if he's a newbie, its not a straight forward search as this is a WRX orientated forum. Not a forester.

a SF5, or classic forester may have similar engine chasis as a GC8, classic WRX, but i can assure you its not 100% perfect swap without some trial and error

so, if its a bottom end engine its a straight swap, but you'll want to either put an aftermarket ecu or tweak your factory ecu, assuming you got a MY01-02 GT, from you username

if you decide to do a complete engine swap, you'd want the complete wiring loom to make life easy. Strubaru is the man to speak to here, who has a MY99 GT SF5 and did a complete Spec C engine swap