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Be careful putting new pistons in the original bore. The top ring will most likely travel higher than the old top ring and I doubt there is no ridge at all on an old cast ring motor like that.

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My thoughts exactly. Need to take ALOT OF CARE w the ridge reamer & get it flush but not past flush all around the perimeter and pull the cutter toward you rather than pushing it around in a circle and changing positions often so you are pulling the cutter just right and I would dingleberryhone for the ring type you choose (single Moly) though I hear cast iron rings seal better if the out of round/taper is way out of spec but I'd prob still go w Moly . Use what you have/get it running/tune/enjoy it then swap for better stuff later. Down time is EDIT read the OP & on balancing I would get a $10 dollar Harbor Freight gram scale & weigh the new piston/pin/ring assy & compare that # to what you get weighing a decarboned piston/pin;/ring assy you remove. It's reciprocating weight which is cut in half in the bobweight formula so it's less critical but you'll have the facts in hand & can make an informed decision and if the new is heavier you can grind the "square" on the rod small end to even it back out as the rod small end is included in the recip weight part of the formula but you'll need to rig up a fixture to hold the rod C to C horizontle while weighing the small end on the ten dollar scale. Just something to consider depending on how far you want to get into it, so much for getting up & running ASAP!