Here’s the thing, properly installed, bolts are stretched every time they are torqued, but only only within the elastic region. They perform like an extension spring. Any bolt that has permanently stretched, yielded, does not return to its original length, is trash. If the rod bolts you have, have only been installed once, they are not likely stretched and could safely be reused. That leaves the nuts. You can safely buy aftermarket nuts. It won’t make the bolt any stronger, but you can eliminate the slotted nut. Very good alloy steel 12 point nuts are a couple of bucks apiece. You need to make sure that the flange on the nut is no larger in diameter than 5/8". Here’s one that would work: https://www.mcmaster.com/90759a350
Great respect for @Rob_Reilly, but am dead serious when I say I would stick that cotter pin in my eye before I put it in a connecting rod. I have found pieces of them in main bearings and once found one in an oil pump, where it had split the stator (outer gear).
90759A350_HIGH-TORQUE 12-POINT FLANGE NUT.PDF (75.8 KB)