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its likely that the combustion chambers are full of oil
I'd suggest to remove the glow plugs and wind the engine over on the starter with some rags in front of the holes to catch what comes out, clean the glow plug tips with some solvent / brake cleaner and then try again
what can happen is it runs off its own oil and this can lead to 'runaway' where the engine revs uncontrollably and can then be damaged by over-speeding
so if this starts be prepared to try and stall the engine by dropping it into gear
the worst case for the knocking could be as bad as a worn big end shell and in which case the engine will have to come apart to replace bearing shells and re-grind the crank
and often this can run to several hundred pounds in parts and machining charges so it can be more economic to replace the whole engine with a good used unit
it may not be this bad and the knocking could just be a lot of pre-ignition due to the oil in the chamber