Next things to check would be to make sure the gap on those plugs is correct, ignition timing, a crack in the distributor cap worn rotor or bad plug wires, faulty emmissions components, weak ignition coil, vacuum leak at the intake, dirty fuel injectors, leaky egr valve.
Not sure what engine you are working on but sometimes you can roll the engine backward and get the gear to disengauge .
Could be the TDC Sensor, sometimes called the crankshaft Sensor.
Worn main bearings sometimes knock when cold starting
Sounds like starter going bad.
the engine will turn over but sometimes wont start. it also sometimes dies while its idling. whats going on
What to do before starting auxiliary engine?
1) the vehicle is upside down 2) ownership has changed 3) common term describing cranking, or starting the engine
If you are asking about the 2.0L engine....the ECM was changed to what is called a NGC and the crank and tone wheel(pickup for crank position sensor) was changed. Also, the blocks are not interchangeable due to the position of the crank sensor was changed. It may be possible to change the crank and and sensor then extend the harness to reach the front of the engine where it was move from the rear starting in 2003 but this would be alot of work. Everything else on the engine should changeover fine.
vacuum leak
If the engine has a timing belt, it should be changed around the 150K mark. If there is a timing chain, it does not have to be changed unless it is damaged.
Changing a valve cover gasket really has nothing to do with an engine starting or not. You need to check your power and gas for problems in starting.
It will last the life of the engine and is changed during a complete engine overhaul.