1999 Mazda Protege hypothetical question

Engine Mechanical problem
1999 Mazda Protege Engine Size unknown Front Wheel Drive Automatic 127000 miles

My car broke down while driving it. The oil light had been going on and off for some time (even though I had an oil change three weeks prior). If you were the mechanic working on this car and discovered that cruddy had prevented good oil from reaching the camshaft and cylinder head (journals were also damaged) as a result the two pieces fused together also causing damage to the timing belt.

What would you have done to repair the problem and why?

If there was cruddy oil in that part of the engine, would it not be reasonable to assume other parts also had cruddy oil?

How difficult would it be to check?

Thanks for your help.