ALL engines are balanced by the manufacturer, just not as well as aftermarket balancing in some cases.
If you look at the webs on the crank, you will see holes drilled in various places this is the balancing.
THey can ONLY balance what you supply them with, basically if it rotates they can balance it, so long as ther is sufficient metal to play around with.
Normally CRANK RODS PISTONS FLYWHEEL are the items that are balanced, rods can be balanced END TO END, then as a whole unit, PISTONS are weight matched or "BALANCED" as the set as close to each other as possible.
Crank and Flywheel are balanced for out of imbalance forces.
These can be done to 1 GRAMME if you want put unless a screamer just not worth it.
The main effect of BALANCING is to make a freer more rev happy engine, for the cost I have always done it on every single engine I have ever done.
It will cause NO HARM AT ALL NOT TO HAVE IT DONE, so don't worry, it just won't be quiet a s smooth as one that has been done.
It is a good idea to have the FLYWHEEL lightened as this will aid ACCELRATION, but not affect power, it does this as it has less RECIPROCATING MASS(engine bits) to accelerate, but can make the engine a little lumpy with less flywheel weight on idle only, but it will defo rev easier.
tabetha