Sounds like catalytic converter is plugged. It is illegal to tamper with the emission system, so the best bet is to purchase a new universal converter. Junkyards typically don't sell used converters for liability reasons.
A direct fit aftermarket converter can cost $200-$400. A universal converter costs $100-$200. The engine is tuned for a certain amount of backpressure, and removal of the backpressure can cause driveability issues.