2000 MX-5: Random misfires and P0420 code

[miatamaze]

This is my first Miata. Purchased it locally from it's second owner just 2 months ago. Only had 54,000 miles, so I figured it was barely broken in. The previous owner said he had just replaced the catalytic converter. After about 2 weeks, I noticed rough idling when first starting the engine. Then the CEL came on with "random misfire". Cleared the code and thought all was well. A few days later...happened again. After several repeat performances, there was a very bad episode were the CEL started flashing and the car could barely move itself. Now there is a very bad rattle coming from the exhaust and when you bang on the converter, it sounds like there is a big rock loose on the inside. From all the reports I've read I think it needs a new coil pack. The previous owner replaced the crankshaft position sensor and installed new NGK plugs and wires before the new converter. I have purchased a new coil pack and will soon attempt to replace the old one.

Does anyone think this is my best option?

[Docbass]

I would first run a compression test just to be sure you have not burned some valves or a piston has a hole in it. Then do a wire and plug change then if the problem is not resolved then do the coil packs.
Then look at replacing the cat again
Let us know what you find to be the problem