What you can do to balance it somewhat and get a so called balance factor of 50-60%. Do this if you got plenty of spare time.
You will need a very good scale, a digital if you got one.
Disassemble your engine and clean the parts (always nice to work with clean parts).
Remove the conrod and weigh the bottom half and then the top part of it, write down the numbers.
Weigh the piston/clips/wristpin as one assembly. Add this weight to the weight of the top half of the rod (conrod). Now you take a pecentage of this (between 50 and 60%, let's say 55%).
Add this 55% to the weight of the bottom half of the rod and machine a piece of brass or steel of this weight to fit over the crankpin.
Now you assemble the crank with this weight and balance it on the ballrace surfaces on two knife edges. If it rotates you will need to grind the heavy side until it does balance. You now have an engine that's balanced to a balance factor of 55%.
The factor is rpm dependant and I don't know what factor is best for a specific rpm, I'm just writing this up from memory. I saw it on a site somewhere.
Any material you can remove to reduce the reciprocating mass, that is the piston/wristpin/top half of rod, will make an improvement on engine performance especially at high rpm. It will also reduce overall stress.
I haven't spent any time on this myself since I don't have a good scale and a lathe.