the daihatsu CB has full circle camshaft bearings which require a good machinist to install replacement shells and perform a line bore on. I only know of 1 machinist in Karachi who invested in this tooling for the CB engine. The name of the shop is Salman Engineering.
If the head is not corroded or damaged then do the line bore and measure the rockers and shafts, also make sure the camshaft is not worn out. The lower end is not that hard, it just has a balancer in it as addition. If your mechanic would have done a good job as recommended by the manufacturer the engine would sound like new.