I would like to suggest something I have seen a number of times in the last 50 years on boats that have been sitting and then brought back to life. The guys have given you a lot of great ideas. If all your diagnostics are correct , you could have a partially plugged exhaust sytem. The engine will idle well, but will have little power, start hard and all 6 cylinders will be hitting with wet looking plugs. Rodents like to crawl thru the prop hub and make a nest of rags, insulation, and or other debris above the carrier and plug the exhaust. (typically mice and or chipmunks). You would think the debris would blow out, but it typically does not. You can pull the prop and get a good light and look up in the exhaust for a restriction. Sometimes you have to pull the GC to be sure. The other tip is the engine will be quieter than normal on a flush device in the driveway. It will not have the cackle or bark we 2 stroke guys love to hear. (when you rev it up) The other scenario from storage is a nest under the hood and the first time you go WOT, the debris is sucked into the reeds and again causes low power, but typically the idle suffers. Know this is off the wall, but have seen it many times on units that have been stored. Hope this helps. if not something to think about. We always used to say you need 4 plus things for an engine to run properly Intake, Compression, Power, Exhaust and a place for the Exhaust to go freely. Thanks Dave IN MN