Hi Munir,
I even don't believe in a cracked fly wheel cog and also assume a faulty starter. Possibly the starter cog wheel is damaged due to being stuck in the fly wheel cog.
Removing the starter motor is a bit tricky.
Instead of a long (1 Meter) extension also a couple of shorter wrench extensions are useful. A cardan joint and a long(!) 10mm Allen-bit are required to remove the upper bolt. Some guys also undo (but not remove!) the two rear aggregate-carrier bolts to lowering the back of the engine to ease meshing of Allen-key and bolt.
As far as I remember the oil filter needs to be dismantled.
Good luck!

Beste Grüße
Tim