Today I got opinion from a couple of more experts in this. The counterbalancers in these engines provide secondary balance. Fiddling around with counterweights only affects primary balance. The secondary imbalance that comes with removing the counterweights might be a problem or not, depending on how the engine is mounted in the frame and the vibrational harmonics of the various parts of the bikes. Removing the counterbalancers has caused problems.
The primary balance is the counterweights. Their weight can be adjusted to provide good primary balance at high rpm. A problem with shaving the balancer weights is to provide primary balance. The counterweights can be drilled and heavy metal plugs interference fit into the holes. The crank could be lightened by drilling around the web near where the big end is. The pistons and rods could be lightened to get primary balance.
I looked at doing these things after work today. The heavy metal inserts will require drilling the counterweights. There is not a lot of room for this. It will be hard to get significant added weight from heavy metal plugs to compensate for metal removal from the counterweights. Drilling balance holes on the crankpin side of the crank weights will weaken them in a critical area. I am not sure how to lighten the rods and pistons.
The bike is raced in FIM and the return run needs to be made within an hour. Once I make the down run I wait for the FIM escort to accompany me back to the impound. Then I go to impound. Adding fuel or fixing things is done there. Any adjustments in impound require me to walk down pit row to the truck, dig out what I need, walk back to impound, fix or adjust, walk back to the truck to put stuff away, and walk back to impound. Then I find the escort, tell him to hurry up, and then we ride down to the other end of the course. Next, I convince the flagger of the urgency of the situation. Almost always, a few minutes before the hour is gone, they flag me off. A smooth running bike that does not vibrate stuff loose is a real asset for an FIM racer. Although I do not have time to monkey with the crank, I would not fiddle with it if I did have the time. It is sort of like a sprint tuned engine vs a motor set up for endurance. A person needs to finish to win.