I'm looking for a replacement piston for our 1939 43 ton GE which is proving very difficult to find. It's a Cummins engine with the serial number and other specs below. The part number is #148960. Any leads, new or used, would be appreciated. Thanks.

If you can't find one, you might be better off sending these people a good sample and refreshing/rebuilding the whole thing. I had Arias make custom pistons for my old BMW race car years ago - great pistons and not that pricey, but I don't think Arias is around any more.
Diamond does forged diesel pistons.

https://diamondracing.net/if-9-custom-p ... -form.html

There is one listed on Ebay. If you can't find one, I have a running take out HBI600 from a 50 ton GE wit the PT fuel system in MI.

Have you tried McHugh? Tom Gears, one of the moderators here, has shared some interesting and helpful commentary about McHugh. I believe GE switchers are one of their specialties.
https://www.mchughlocomotive.com/