The C-123 used Pratt & Whitney R-2800 engines, so I would suggest looking for some freeware sets, like those for the F4U Corsair. Just make sure you get a set with an electric starter sound, there is a set (primarily for the CFS users) that uses a cartridge starter sound. I used a few different R-2800 sounds to cobble together a set when I was using the C-123 in FS9.
