Just to clarify, the dealer thinks that the exhaust is blocked NOT the catalytic converter. I live near the beach and most metal rusts quickly.
Also he said that the sensor is required for the fuel injection computer? But where is the sensor? if I cut the exhaust pipe just before the exhaust box I can leave the sensor where it is, as I assume it is closer to the engine?
If the original exhaust is in your possession you can just look for an open threaded plug on the silencer itself, or an open threaded plug on the pipe somewhere between the silencer and the flange which bolts to the cylinder head.
The sensor might still be bolted to the plug, which obviously means the plug might not be open.
If the original exhaust is not in your possession, you could have a close look at the current exhaust mounted to the scooter.
If you cant find anything, I think there should be no reason to worry about a sensor - I think in such a case there is a possibility that the mechanic was thumb-sucking about a catalytic converter in the exhaust system.
My scooter (Agility 125) has an EGR system which control emissions in a different way. I think there is a possibility that your scooter also has an EGR system - instead of a catalytic converter.
I just Googled the subject, and had a quick look at Youtube videos as well. I could not find any evidence of scooters having catalytic converters.
Maybe you could have a look at the video below. Towards the end of the video it shows the components of an EGR system on a smaller scooter. The system on the video's slightly differs from the EGR system on my scooter - some of the components are located under my scooter’s floor panel.
If your scooter also has an EGR system - I think the EGR valve (bolted to the tappet cover) probably got stuck. In such a case the exhaust installed by the mechanic will probably only be a short term solution - the exhaust will get clogged again.
Like I mentioned - I think you should determine whether your scooter has an EGR system like this, instead of a catalytic converter: