On the subject of dynamic balancing, the flywheels on AMC cranks are seperated by the centre main bearing & each flywheel has a different set of balance drillings, as in different position, depth, & number. If the crank had been statically balanced this would not be the case.
That might be worth investigating, historically speaking - but could a previous owner have done that ?
Enfields claimed that they alone were doing dynamic balancing of their twin cranks, and it was considered sorta new technology at the time, for brit twins anyway.
AMC in later years reportedly had a less than perfect record with Norton engines, it turned out the chap doing it was just drilling the same pattern of holes on everything,
without actually individually testing the new cranks. Or so the story goes.....
Piston weights are not some holy grail, I've got some engines with IRON pistons in them (although not parallel twins), although 3 extra ounces per piston does sound like
a bit, it will just shift your balance factor to a few % lower.
A few new holes in your crank would counteract the new piston weights, if you want to keep the same % factor - if there is room for them ??
Or some mallory weights.