The rotary power switch may be a simple 1 pole, 6 way, in which case it would be fairly easy to source a generic one.
If it is more complex than that, it may have to be replaced with a vendor-specific part. I would suggest checking how the wiring is arranged, look for manufacturer names and part numbers on the switch, and maybe post some pics of it on here.
The power switch is a heavy duty relay known as a Contactor. First step is to determine the voltage of the operating coil - it may be 12v, 24v, 42v, or mains - should be marked. Then check how many contacts it switches - and how many are actually used. Most contactors are designed to handle 3 phase, with 3 pairs of contacts, but a single-phase welder may only use 1 pair of them ( or spread the load across all 3 pairs wired in parallel ). Again, post some pics here.
