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I would highly recommend taking the engine to a Evinrude dealer with experience in the 4 strokes. If there is not an Evinrude dealer around, take it to a Suzuki dealer as it is in reality a Suzuki.
You seem to have gotten hold of some substandard mechanics. You either have compression or you don't. It doesn't come and go.
I have never seen an ECU go bad where the engine ran but no diagnostics.
I would kind of agree about a head gasket but before replacing it you need to have a leak down test done.
Your engine only has 2 coils. It is a lost spark system which means the two cylinders to that coil fire at the same time.
You need to do a spark test to see which coil is not firing. You can use a timing light while running to see if it is firing or not.
Finally I would monitor the engine with the computer diagnostics which can tell you if the MAP sensor is actually bad. Many times a code is temporarily set due to messing with the engine and tests.
Let me know what you get to these checks and what you find and we can go from there. Let me know if you have additional questions or how we can go from here.