I picked up an old Lincoln AC-225-S a while back & have been using it for a few small projects. I finally got around to replacing what appeared old enough to be original leads (maybe...). After getting it home I had (re-)patched them in multiple places with self-fusing tape w/an over-wrap of vinyl tape before using it but the old stuff really needed replacing.
I picked up some new lead wire from WeldingSupply.com. I went with 4 AWG as that appeared to be what the original size (also each new lead is only about 18' & I typically run pretty small rod). I also decided to do a small upgrade & added Dinse style panel connect sockets, with mating connectors on the leads. The Dinse connectors came from Amazon.com. If I need heavier/longer leads, with the disconnects it will be easy to do a swap out. I crimped lugs on the internal replacement cables between welder & panel mount connectors.
While I had it open, I dosed the switch contacts with DeoxIT-D contact cleaner & worked the switch around all positions a few times to help clean & lube them up a bit. Sorry, but I didn't think to get any pics before things were all closed up.