The first step would be to get hold of a wiring diagram or atleast a pinout for the ecu that your engine is running on. You can search for it at Toyota Yaris Forums, Photos, Videos, News, and Discussion - YarisWorld by entering the engine code, 1sz-fe in the search fields. If you cannot find the pin out, you can open up the ecu and the pinout will be printed on the pcb aka the circuit board. You can draw the pin out on a piece of paper. Once you have a hold of the pin out, you need to check the continuity of each wire exiting the ecu to the respective sensor to ensure that the engine side wiring is intact.
Once the engine side wiring is done, you'll need to focus on the body side wiring because that is where the ustaad would cut corners. Chances are, he'll only connect 2-3 wires i.e. the power and ground and will hand over the car to you. You'll need to rectify the body side wiring for the swap to be a pleasant experience. To do all this, you need to have the pinout handy.