It's theoretically possible to take a car long block and convert it for marine use by swapping over the marine-specific components, but there are a few things to consider. Marine engines are built to withstand harsher environments and have key differences, including stronger internal components...
You might also want to check the cylinder head gasket for any leaks or signs of damage around cylinder 4. A small leak could cause issues at idle but might not be as noticeable at higher RPMs.
Firstly, even though your compression is good, there might still be an issue with the fuel delivery to cylinder 4. You could double-check the carb rebuild for that cylinder and ensure all passages are clear. Also, consider checking the fuel pump and lines for any blockages or issues that might...
Oh, that looks like a Halibut, and it's indeed a delicacy! I've had the pleasure of trying it at a local seafood shop, and it was superb. I can see the resemblance with Flounder we have in the Gulf of Mexico; they must be from the same gene pool. Flounder may be smaller, around 1-2 pounds, but...