Subaru engines are pretty tender…It doesn’t take much abuse to kill them… Aluminum engines in general do not take kindly to overheating…The car is 12 years old, maybe it’s time to move on…
Usually, I would recommend installing a used engine from a salvage yard, but with a Subaru, finding a decent one may prove difficult…For a while, you could find used engines imported from Japan where they total cars for almost any minor accident and they are not permitted to sell salvaged parts…So they export them to the ready market in the States…See if you can find one. I feel that any attempt to rebuild your engine would be disappointing…