Keith.
Have never balanced a 2-stroke crank but, have balanced many 4-stroke cranks. A crank is a crank.
You cannot put a crank into perfect balance. Reason being is you are dealing with two completly different forces. Rotating weight and reciprocating weight.
The rotating weight being the crank, flywheels, and the bottom half of the connecting rod, (all the parts that go round and round). The reciprocating weight is the piston, rings, wrist pin and the top half of the connecting rod, (all the parts that go up and down). The flywheel does not matter since it is a balanced member on its own.
As cujet pointed out, you must use a balance factor. You must determine what RPM you want the engine to run its smoothest. You will still have vibration below that RPM and above that RPM but, when the engine reaches its working RPM it will smooth out.
If you only had a rotating mass, then you could balance the engine perfectly at all RPM's. But, the recriprocating mass, (the up and down weight) is the one that throws a wrench into the works. You have to use a formula for this and a percentage factor.
Inorder to balance any engine, you have to completly dissasemble the crankshaft, rod, etc. and weigh each part, rod, piston, rings, circlips, piston pin, etc. Once you determine the proper balance, you have to either grind or drill into your crankshaft flywheels to put it into proper balance.
If your interested in balancing your engine let me know, I'll PM you the formula.
Also, if you are having problems with excessive vibration, I would first check a few things like loose engine mount bolts or a cracked mount. Most engines come balanced pretty well from the factory. If your crank assembly has ever been rebuilt using an aftermarket rod kit or you are using an aftermarket piston, your balance could be out a little.
Just my $ .02
Ol'89r