2013 Outback – Mystery Misfire

My 2013 Subaru Outback w/ 140k miles was burning 1 qt of oil every 300-400 miles, misfiring occasionally, losing power, sometimes stalling out on hills, and of course burning through catalytic converters. I took it to a couple local shops and they said “Subarus just burn oil”

So, I replaced the engine with an engine from a 2019 Legacy. The new engine has 20k miles on it. I basically replaced the long block and kept most everything else from the 2013 in order to keep the computer happy. I had to modify the TGV system and swap the camshaft sprockets. This engine has a timing chain, not a belt. Now it doesn’t burn oil, but there’s a misfire. Not sure if it’s the same misfire as before, because it was very intermittent on the old engine. Took it to a mechanic who did a vacuum reading and it was a little low, did a compression test and it looked pretty normal. The misfire is mostly cylinder 1, with occasional other misfires on other cylinders. I’ve replaced spark plugs, replaced battery, swapped coil packs, swapped fuel injectors, monitored fuel trims (completely normal), and adjusted valve lash. I took it to another mechanic who said it’s too complicated and they don’t want to see the car again.

Where do I start looking for the cause of this misfire? Fuel trims are almost always below 5% in either direction, and have never exceeded 10%. I’m inclined to check the electrical for corrosion next, but I don’t really know. Would it throw a code if there was corrosion in the wire going to the cylinder 1 coil pack? I realized there may be a different part number for the crankshaft sprocket between the 2 engines (I didn’t replace that so the sprocket is from 2019) but it seems like a timing problem would cause misfires equally in all cylinders? Anyway, if anyone has tips on what to do next, I’d love to know them. I have a multimeter but not a scope. Thanks!