Without being rude...if whoever rebuilt the engine is a "qualified" mechanic etc....then finding the problem really shouldnt be difficult when he has the engine/car in front of him.

But either way, the valves are not closing correctly that is blatantly obvious from symptoms described, and lifters not getting any oil wouldnt have any impact on that

Whether cams are not installed correctly, valve seats cut too deep, wrong length valves, wrong lifters etc etc
Again, all basic stuff that he can check although he may need to remove the head again.

But also stuff that should really have been checked before installing the head in the first place.