To elaborate on what Roland said.
Most replacement rods/pistons try to maintain factory weight.
As these little engines see only 36-4000 rpm's
most, cannot tell any difference if there was a slight difference in replacement rod/piston weight.
Now if the overbore was +.125 or a different material say like from old skool cast iron pistons to aluminum pistons, then there would be a benefit to re balancing of the rotating mass, or if it is going to see 7 or 8000 rpm's then it would be recommended, to keep all parts inside the block, as intended.
An example would be the once popular John Deere 2 cylinder "johnny poppers"
The early ones used cast iron pistons, later models went to aluminum pistons
and they easily were bored 1/8" oversize.
Those 2 pistons were big @ 5-1/2" and more, diameter (coffee can size)
But they didn't really need a re balance as the rated rpm's were approx 1200.
If one were to investigate the rod failures in the Kohler cast iron singles,
They would see most are from lube failure, (poor maint/ periodic oil changes) abuse, like over-speed.
Or continuing to operate with obvious knocking sound till destruction internally.
Occasionally a rod will fail just because it had a structural imperfection but it is an exception rather than the rule.
Ya I know,
too much info.
