Your permanent magnet electric motor can be used as a generator. All that needs to be done is to apply an external power source to spin the shaft of any permanent magnet electric motor and your motor becomes a generator. Your electric motor generator will need to be spun slightly faster than a unit specifically designed as a generator, but there is little difference between a permanent magnet electric motor and a generator.
Cut one of your 6-foot 2 by 4s in half so you have two pieces each that are 3 feet long. Cut two pieces from your other 6-foot 2 by 4 that are 2 feet 9 inches long each.
Set the cut 2 by 4s on edge and arrange then so they form a square 3-foot by 3-foot frame. Using two 2 1/2-inch wood screws at each corner secure your frame. Lay the 3-foot X 3-foot sheet of 3/4 plywood on top of the frame and screw 3 1 1/2-inch wood screws along each edge. You now have a very sturdy platform on which to mount your permanent magnet electric motor and your small gasoline engine.
Set your small gasoline engine in place on one edge of your platform. Drill mounting holes and use four of your 1 1/2-inch bolts (with nuts and lock washers) to secure your gas engine to your platform. Make sure that the shaft of your gasoline engine is set so that it will be parallel with the shaft of your permanent magnet electric motor when it is mounted to the platform.
Attach one of your pulleys (4 to 6 inches inches in diameter) to the shaft of the gasoline engine that you have just mounted to the platform. Attach the other pulley to the shaft of the permanent magnet electric motor. You will need an Allen wrench or a screwdriver to attach the pulleys.
Set the electric motor on the platform so the two pulleys line up and put a fan belt around the pulleys so that when the gasoline engine runs the belt will cause the shaft of your electric motor to spin. Pull the electric motor as far from the gasoline engine as you can (keeping the pulleys lined up) so that the fan belt is stretched tightly between the two. Mark where you need to drill your holes in order to secure the electric motor to the platform. Set the electric motor aside.
Drill holes where the electric motor will be mounted to the platform. Use your jigsaw to elongate the holes into slots that are no wider than the holes you drilled. The slots should be perpendicular to the shaft of the gasoline motor that is mounted to the platform. The slots allow you to position the electric motor at slightly different distances from the stationary gasoline engine, which allows you to tighten or loosen the tension on the fan belt between the two. Position the electric motor so the fan belt feels tight and then secure the motor in place with four 1 1/2-inch bolts with lock washers and nuts.
Separate the wires that originally sent power into the electric motor. Start the gasoline motor and allow it to spin the shaft of the electric motor. Do not touch the wires that originally fed power into the motor as these wires will now be charged with DC current.