first thing i would do is check the batteries
make sure the water level is above the cell plates, make sure they are charged, do a load test on each battery and make sure they are holding a good load and not dropping more than 1 volt on a 15 second load test. Make sure all the cables and terminals are in good condition, no corrosion, if corroded clean them, if any cables have too much corrosion inside the cable replace the cable. Once you verify the batteries look at the brushes in the motor if you have access to view them, make sure they are not worn out. The brush should be protruding above the top of brush holder and the armature should not look burned. (some of these motors have a cover over the brush access windows on the brush end of the motor and may require the removal of some screws or maybe it might have rivets holding it on).