Yah, it should be replaced at ~100,000 miles or so. Its a major operation because you have to remove a lot of stuff bolted to the motor before you can get the plate off. You also have to take the rocker cover off at the same time, good time to do lift bolts. The dealer says you must do the chain, guide & two cam gears which are very expensive parts.
I also have a broken AIC. So I decided to change the motor instead, its cheaper & more effective.
Celicas have inteference heads, if your chain goes the valves slam into the pistons & your motor is done. So not changing it could cost you a motor.