One thing that can cause a DTC P0300 is a stretched timing chain.
On your engine, remove the distributor cap.
With someone turning the engine over by hand at the crank bolt in the normal direction, watch the rotor in the distributor rotate.
Now have that person turn the engine over in the opposite direction while watching the rotor.
If the engine can be turned over in the opposite direction more than 5 degrees before the rotor in the distributor begins to rotate, that indicates a stretched timing chain.
Tester