2000 Subaru Outback engine

I believe all 2000-2004 harnesses should match up. You may want to chase the matching ECM from the donor motor, but many interchange easily. It's important to find one with dry heads indicating a prior head gasket replacement. Remove the front left cover to inspect t-belt condition as well. If unsure, I'd probably replace the t-belt just so to replace the old idler and tension pulleys at the same time, as they indeed can fail. The odds are pretty unlikely that you'll get a used motor that knocks badly from connecting rod failure, but it's the risk. Again, if the heads are dry the odds are that they're not warped, the HGs are ok, and you're probably good to go. Salvage yards will know the swap potential for the years. But I have to ask: are the rear quarter panels corrosion free enough to support a heart transplant? What about that 20 year old 4EAT? Make sure you drain and refill it a couple of times and hope its internal seals are still supple enough to hold pressure. Many have dried up by now and result in violent "slamming" in D or R after longish delays, surprising their drivers by knocking over pedestrians etc. SAo check that old tranny too. Good luck. Ern (Disclaimer: I service and sell 2016+ OBs and Legs in Boston)