Problem with knocking sound on engine~

Needed some advice of where/what should i do..kinda out of idea where to troubleshoot.
Engine:-
CD20(850) /almost identical to CB41 but its carburated

Problem:-
~really loud knocking sound from engine

Job Performed:-
~Check Valve
~Changed new oil pump(this where it starts,but doesnt fix the problem that arise)
~serviced

Now,taken apart front cover/pulley..looking for the place where the knocking sounds begin..found some loose spots..but need more info on this engine,this problem started with oil leaks then borrowed by my cousin for few weeks,when i get back..it comes back with white smoke and knocking =.=!