P0422 Fault in 2000 Kia Sportage: Catalyst Efficiency Issue & DIY Solutions

I own a 2000 Kia Sportage with approximately 80,000 miles on the odometer and have recently started receiving a P0422 diagnostic trouble code. This code indicates that the catalytic converter in Bank 1 is not operating efficiently. The vehicle runs smoothly, idles well under load, and I’ve noticed no performance issues or any rotten egg smell from the exhaust pipe—this is consistent with a catalyst-related problem rather than engine misfires. I've researched common causes and learned that a clogged pre-cat (pre-catalyst) is frequently responsible. However, I'm hesitant to replace the expensive catalytic converter at this time. Before doing so, I'd like to understand if there are any other potential diagnostics or low-cost troubleshooting steps I can take. Additionally, I’m wondering whether it’s feasible to install a universal-fit catalytic converter as an adapter in place of the pre-cat. Specifically, does the exhaust opening from the manifold need to be large enough for proper gas flow and conversion? If not, would such a modification compromise emissions compliance or trigger further errors? I'm looking for reliable, cost-effective solutions before investing in a full replacement. Any advice on what to check first—such as wiring connections, oxygen sensors, or other components—would be greatly appreciated.