Tazz is right; quite often you can get within 10 grams by selecting a heavy rod with a light piston and vice versa then find scale and grind away; although I'd tend to remove material only from the boss on the conrod, when I did fuel quality control for the Aussie airforce I used Sartorious scales accurate to 1/100 of a mg perhaps a local airport can help out, worth a try.
Dynamic balance for race conditions on your rotating mass is a speciality that most shops will not be equipped to handle so may I suggest you put up a new post asking if there is a reliable race engine machinist within coo ee (Aussie term for local-ish) of you the mail service should be able to deliver such a parcel. BTW your engine is basically an air pump so you may need more cam. fuel. different manifolding etc, piston dwell is a good place to start though.
Cheers from Aus