I`m very suspicious of the mechanic performiing the work. He may be trying to pull the wool over your eyes. The "POP" sounds more like you may have had a rocker arm come loose and/or a pushrod,allowing a lifter to come out of it`s bore.
I would double check ALL of the parts that the mechanic says were faulty.
If the timing cover is undamaged, he`s feeding you a load of crap.
I know I shouldn`t overgeneralize, but I`ve had "mechanics" try to pull crap on me in the past. They weren`t aware that I knew more about cars than they apparently did. I just didn`t have time to do the work myself.