Legos. Subarus are Legos. Any motor and transmission combo from any other Impreza/Legacy/Forester will bolt right up. The only difficulty is wiring it all together. The best and simplest solution is to buy your engine/transmission combo with the complete wiring harness from the same car. Then you just yank out all your stock stuff, and replace it with the new stuff. It's labor intensive, but there's no real serious problem solving or head scratching to worry about if you do it that way.