Okay, yes those flywheels are on there pretty good. But not because of being seized up. It is how they all are because of their tapered shaft. Read some of the folks on here with their efforts in removing the flywheels. Secondly, how did you verify one cylinder wasn't firing? When it is running can you take one spark plug boot off and the engine changes RPMs? And the do the same with the other. A non-sparking cylinder will not make any difference. But if both change the RPMs then you have another problem. However if you already determined a bad or non-sparking in one cylinder, then swap the spark plug coil and see it the problem follows that coil. If it doesn't then you have internal issues. If it does, then you need a new coil...