What is the cause of Choking Emission

The rattling sound comes somewhere under the hood, low compression, very rough idling and slight vibration at take off. It's like the sound you hear when you select a high gear for a slow moving car. I've replaced the engine of the car yet, the problem persists. The truth is that I have no mechanic that knows how to do solve my problem.

By choking emission I mean, the smoke from the tail pipe has the choking smell of burning metals.

I think the catalytic converters are bad. Is there anything wrong if I change them with those of Japanese cars? They are very expensive in this part of the globe.

Thank you for your patience and untiring effort to render valuable advice to help me.