Sludge would clog the pump and the pressure regulator that is inside the pump.
I don't see any reason to replace a tank unless it is leaking.
1. Do you have a filter between the tank and the furnace?
They don't cost much.
2. It is probably either a plugged nozzle or a bad electrode.
Both can be fixed or replaced for very little cost.
Did he replace the nozzle and/or check the electrode?
Since the boiler smoked it is most likely a nozzle or dirty electrode.