Tim,
I've seen welders where the wire feed motor control circuit has gone kaput. It's normally the motor's speed control transistor or the (smaller) transistor that drives the base of this transistor, if you know what I mean
However, if, as you say , the arc has gone as well, it could be the hefty rectifiers (bolted to a heatsink) that may need attention. Have a look at the circuit board and use your senses... the nose is good at detecting a burnt out component if it's not obvious by sight!!
Good luck, and be careful if you're having a look inside when it's plugged in!
Trev