Toyota Repair: engine clacking in an 87 toyota celica gts, toyota celica gts, toyota celica

Question
when i start the car the engine clacks like it is going to blow up.  there is a rattle in the motor.  Last time this happened it ended up being a spark plug that had been hit and broken.  we just replaced it and it was fine.  I also haven't given the car a tune up in two years.  what could be wrong with it this time?  would a tune up help.  and why would changing a spark plug last time aleviate the problem?

Answer
There seems to be a serious problem with the engine, whatever hit the spark plug could be the cause, how long ago did this happen?
The rattle noise in the engine is a cause for concern, without hearing the noise myself I really have no way to diagnose it, it could be just a worn timing belt or water pump or a connecting rod noise which means serious engine damage.
I would advise having a toyota dealer take a look at it.