There's a lot more to lightening a piston to obtain a balanced engine than drilling a bunch of holes in it... Basically, the only real way to do it is to have the crank out and add a bob weight then set the crank on a set of leveled balance knives, add or reduce weight on the bob weight until the crank will rest at any position since it will always try to rest with the heavy spot down when sitting on the balance knives.
Once you got it balanced you need to weigh the bob weight and record that number.
This is what your piston should weigh... Nope... not quite... if it was a twin cylinder engine, that would work, but not so well on a single. There's what's called a balance factor where the piston should weigh a certain percent of the bob weight's weight. This is done to compensate for the piston acceleration that would throw a perfectly balanced assembly way off balance so a balance factor must be used to help offset this.
The next big issue with single cylinder balancing is that no matter what's done it'll never obtain a perfect balbnce, but it can be balanced to run really smooth in a certain rpm range so basically if you got a fast engine that's going to be run mostly at a higher rpm then you balance the engine so it smooths out at this rpm, it'll still vibrate progressively harder as the rpm increases above or decreases below this rpm, but there's a fairly wide range it'll stay acceptably smooth... There's more to this than I can explain, but the info is out there with a good amount of info in this forum as well as youtube for balancing these engines.
About the best we can do here is get the crank straight and true, then find out what weight others have lightened their pistons to and this will get you a lot closer than running a stock weight piston, but if you copy someone else's numbers, make sure their engine is turning the same rpm's yours is at cruise speed. The typical concensus here is to reduce the weight of the piston, but you can still go too far with it and make things worse.
I would recommend looking up posts and articles on balance factors and also check youtube for balancing how tos on these engines if you want to get it as precise as possible.
