Are you sure you want to rule out taking the head off and inspecting/trying to repair it? Even if you still end up getting a new engine your old one will be worth something second hand and the cost of that work may be recoverable eventually, especially if you get the engine running. Maybe you find any damage is limited to the head? I'm not familar with that engine but any chance it has cylinder liners that can be changed?
Maybe also consider getting the engine running or putting the new one in and straight away measure the exhaust back pressure. If it is indeed too high then you also have a reference for how much you can reduce it with changed muffler/shorter hose/larger diameter hose. I haven't read all these posts but maybe part of the problem might be the previous owner leaving the boat for extended periods without running the engine and the problem won't re-occur with more regular running and monitoring?
Regarding isolation, some engines are permanently grounded and others have a relay that activates via the ignition otherwise isolates them when they're not running.
John. (Maramu #91 Yanmar 75HP, plus 2 other boats - 1 MAN 1000HP. 2 Yanmar 650HP)