Leaking injector is the most common cause.
a hole in one of the pistons
There are several things that can cause diesel fuel to get into your crankcase. A broken piston ring is the most common cause.
No.
No , it would crank , but if there was no power getting to the electric fuel pump it would not start
in the crank case
There are several reasons that will cause an engine to crank over but not start. The most common cause is a lack of fuel.
could be many things, probably a crank sensor
fuel pump was coming on before changed fuel filter now its not is there a reason for this
Have you checked for fuel and spark? Camshaft sensor and or crank sensor will cause that problem as well!
That depends on your moped. Most mopeds madein the 80's are two stroke engines. Which do not use oil in the crank case. the oil is mixed in with the fuel.
Depending what year, make and model you have, yes.