Electrical contact cleaner. You can try the spray can type but better would be the liquid quart bottle from Affiliated Electronics in Tulsa (see website) on Admiral. Good stuff. Tell them what you want to do as they have two or three different types.
But...since you're working on an electrical component switching ~50-amps (never switch under load, but that big transformer pulls some just idling), I'd want it to work correctly instead of making a big flash with my hand on it.
Any Lincoln dealer (I trade at AirGas SouthWest) can get the part or try the Lincoln corporate sales office in Tulsa.