No I'm not talking about truing, I'm talking about balance. If you replace half of the crank with another half you could be increasing or decreasing the total assembled counter balance weight from the variations in crank web weights. The reciprocating mass can be assumed to remain constant because your using the same weight piston and rod. Maybe it's a non issue but that would be highly dependent on how tight the manufactures tolerances are when they manufacture the cranks. A few grams could make a significant impact on the natural frequency of the rotating mass. I understand the engine won't be truly balanced for all RPMs.
Thanks for the link I'll read.