Your mechanic is partly correct. The 1990 engine will bolt in, but you must change the throttle body, distributor, and Idle Air Controller. You will use the engine wiring harness from your 1989 car. While the Coolant Temperature Sensor is different, the LH 2.4 Coolant Temperature Sensor works fine on LH 2.2 engines. The later injectors are also different, but they work fine on the LH 2.2 engines.

There is no reason to change exhaust or intake manifolds unless you buy an engine without turbo. In that case you will move the exhaust manifold and turbo assembly to the replacement engine.
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john