To determine the connections on the existing MOSFET trace the wiring through the tracks on the printed circuit.
One wire from the battery should go (Possibly via a switch and/or fuse) to one pin on the MOSFET. If that wire is negative then the pin is the SOURCE, or if the wire is positive then the pin is the DRAIN.
Another MOSFET pin will go to one of the motor terminals, if it connects to the positve motor terminal then the pin is the SOURCE, otherwise it is the DRAIN.
The third pin is the GATE, this will not be connected directly to the motor or battery but will be connected to one or more other components.