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Spark, Fuel, Air, Compression. The four basics to check prior to going on a deep dive. Agreed that it’s not the carburetor nor the ignition with your swap testing. You installed a known working carburetor and magplate on the problem motor and it still doesn’t work.
Have you verified that fuel is actually getting to the carburetor? The sintered tank filters in the OMC classic motors are notorious for getting clogged when they get stored with fuel mix in the tank, then sit for decades. Removing/cleaning the tank filter, fuel lines, and of course the tank, is a necessity on these 3 hp motors.
Tubs has some good advice regarding the rings possibly being stuck. If you do a compression test and the compression is 70 psi or better in both cylinders then compression is ok. Removing the exhaust cover will likely cost you a gasket, but a new gasket, and resurfaced exhaust cover is a wonderful thing.
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