what sort of noise is it? tinging, rumbling, banging?
most of the time mechanics dont measure the wear on the rocker arms and their shafts, sometimes they even install the shafts wrong, one is longer than the other, installing them wrong causes loss of adequate lubrication to the rockers and they can quickly wear out.
the daihatsu CB engine has a very unique operating sound, it has a very soft nudging sound as you accelerate the engine with bonnet open, at mid and high rpm it has a machine like sound - pretty quiet as its iron block.
before any guesswork, connect an oil pressure gauge in place of the oil pressure switch and test oil pressure at cold and hot engine.
also keep in mind that if your engine is not working at normal temperature which is minimum 82C - then all the money you have spent will be wasted in a few months.