i don't have pics at the mo, but a "decat" refers to the exhaust downpipe. In stock form, starlets have a cast downpipe which houses a catalytic convertor with two lamda sensors, one before the catalytic convertor and one after.
The downpipe is located in the engine bay, to the right of the turbo in front of the radiator. a standard downpipe will be a rusty brown colour with several heatshields covering various sections.
a "decat" is an aftermarket downpipe (usually stainless steel) with the catalytic convertor removed and the bottom lamda sensor removed.
This is to increase the flow of exhaust gasses, thus reducing turbo spool up time