I expect your rod and bearings are OK. Can you post some pics of your piston and/or cyl bore? Parts are not easy to find new old stock (NOS) however they do show up on ebay once in a while. Used in good condition is easier to locate. Here is a P&C set for "Best Offer" on ebay.
https://www.ebay.com/itm/JONSEREDS-...a=1&pg=2047675&_trksid=p2047675.c100005.m1851
Not a bad price.....and with a new ring would likely work perfectly. However fixing the P&C is necessary, locating the reason the original failed is crucial......otherwise the same ending will happen to the new P&C. You likely have a bad crank seal which I would replace both of out of hand when rebuilding most any saw. Oil ratios are kind out the window. The ratio stated of, say 24:1 was intended for the oils currently available a the time of the saws manufacture. Oil technology has improved leaps and bounds since the '70's....provided you buy good quality synthetic oil. I use Stihl Ultra exclusively at 50:1 in all my old Jonsereds from the 49SP to the 111S and have never scored a piston. But any good quality synthetic oil other than Stihl is likely as good or better. My point is there is probably an underlying issue with the saw whether crank seals, leaking intake gaskets, leaking cyl base gasket or the carb simply adjusted to lean.....it's always better/safer to be adjust a bit to rich rather than any to lean.