I think you have done the right thing, and there is a good chance you have fixed your knock - I can't tell from the limited evidence that has been posted. I'll try to clarify what is going on, Mark. Look at the upper shell of the big end bearing of the rod that was in the mower. It appears to be worn through to the backing on one side, and the wear pattern does not even reach to the other side. This means the connecting rod is bent: the gudgeon pin hole (or little end) is not parallel to the crankpin hole (or big end). As a result big end lubrication has been a bit poor, and equally importantly, the contact force has been concentrated on the outer edge of the bearing shell, causing excessive bearing pressure, and physical contact with the crank pin at times, leading to wear. Compare this with the bearing shells of the other engine: the wear pattern there is just about ideal, except for not extending all the way around the shell. In other words that rod is not bent. The bearing may have done a lot of work and gradually accumulated some normal wear, but it is still a healthy bearing.
What we don't know, is whether the failing, unhealthy bearing that was in your mower engine, has already generated enough slack to be making a loud noise. The only way to find that out would be with Plastigage: because the rod bearing has now worn tapered due to the misalignment, it isn't completely practical to measure it with a micrometer. So, you have made the right move by replacing the bent rod with the straight one from the other engine, and temporarily using the bearing shells from the other engine's rod. (They were already in the rod, and are therefore now in the engine.) If your knock was from that worn, tapered big end bearing, this should fix it. If the knock was external to the engine, of course it won't fix it, but you will still have fixed a significant fault in the engine. One day, when the engine is in need of overhaul, you could fit new bearing shells, but it doesn't seem an urgent issue to me. Now you can start the engine, and see if it runs properly and quietly. If it does, but there is a knock when you put it back on the mower, then you can find the external problem causing it. In the meantime, while isolating the cause of the knock you will also have fixed a developing problem in the engine, so your time has not been wasted.
To get the smoky engine ready for service you need a good rod for it. I suggest you should not fit the bent one from the original engine. If the bearing shells are still in the rod from your remaining engine, you can check the wear pattern and see how it looks. You might post pictures of the shells here: it will improve this thread, and who knows, we may be able to help you figure out how things were going in that engine during its lifetime.
