Sometimes they do external balance because of the crankcase size limits, sometimes to save on changing crank counterweight dimensions, sometimes because the needed counterweight mass just won't work. You do not grind, drill, or otherwise sodomize the parts except to bring each into balance with the lightest of the parts, in other words make al the pistons the same, rods ends the same etc. This isn't much needed on modern decent parts, any benefit of reducing the sub-gram difference between pistons etc is outweighed by adding stress risers or otherwise mucking up an engineered surface. Old school OEM parts often had pads specifically for balance grinding, ie rods.

That 30gm difference is significant, could be more or less depending on if the mass difference is in the reciprocating or rotating parts. S/F....Ken M