Knocking is only on regular fuel?
What sort of knock is this? Does the engine hesitate a little or just knocking sound. Do you feel loss of power?
Did electrician connected all original wirings back after removing cng? Make sure there are no loose TPS/MAP/ignition coil connector/wirings. Switching wiring to petrol and cng is quite easy, you can do it yourself and do a better job than any mechanic. It hardly takes 4 hours.
When you removed cng, did you removed venturi? did the mechanic seal all holes on intake? Make sure there are no vacuum leaks (hissing sound near intake manifold area)
Are you sure knock is from engine? may be one of your mounting is broken.
Timing for cng is controlled through advancer so when you disconnect it and connect back the TPS and MAP wiring, the timing should be back on factory default.
Timing for petrol is controlled though timing belt. When was the last time you changed it? There are marks engraved and mechanic just has to align those marks to get the factory spec timing.
iirc tank capacity is 45litre, reserve turns on at around 6-8litres