Sounds like it was more than just the FET that blew originally.
If it was OK without the battery power (you say you checked the trigger sensor so I'd say there no issue there), then it's more likely somewhere in the power stage. Either the motor is pulling too much current for the FET you've fitted, there's another separate component issue, the motor is goosed, the wiring has an issue (I know you say you've changed it but that doesn't mean it doesn't still have a fault) or you've created a new board issue in trying to repair it. Without a full circuit diagram and the thing on a bench in front of me I can't really help any more than that.