Not sure about bearings, it's unlikely, but from what you're describing, I would first check your plug (if you havent already done so), then take off the head and check the ring and sleeve. Problems with both of these could cause black oil, and are more likely the culprit, especially on an engine that hasn't been used in a while. If the engine has been stored properly, those issues are rare, but they CAN happen. Also, you don't mention if you changed your reed valves when you changed the carb. Those things are delicate, and even a tiny chip or crack can cause serious problems of your engine, especially if bits of them made it into the cylinder, causing black oil. If it were bearings going bad, you would know for sure. While turning with your hand, it would be rough, and there would be an unmistakeable vibration sound as. From what you describe, something is affecting your fuel flow and burn which would indicate a blockage or leak somewhere. The black oil leaking from your stacks? Check it closely for tiny metal bits. Might be bearings, but most likely piston/sleeve shavings if present.

Just my 2 cents and experience