Its unlikely to be the Cat itself if its overheating, for it to overheat it needs fuel and so something must be wrong that's making the mixture rich. The engine management warning light would normally come on if this is the case - check the bulb is working when you switch on.
Its much more likely its just a bit of plastic on the exhaust somewhere or a possibly a shortage of coolant in the cooling system.
Also some oils can be very smelly and even a small amount dripped/dripping onto some hot bits can take days to burn off. Worst culprit are Castrol Oils which seem to wronk much more than most others.
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