The easiest thing you can try is getting a 2kw 240v to 120v transformer. Yes, a real transformer, not the diode BS. The diode will not give you a true speed reduction, instead it will surge high current, full voltage into the motor at 50% the phase cycle. In this case, with no mechanical load on the motor, the motor will spin at around full speed. A real transformer will deliver a true half voltage. This does not guarantee half RPM, however, the speed will be reduced, but, also, the motor's strength will also be cut.
Check if you can adjust the brush orientation on the motor. In some old DC motors, I use to be able to slightly rotate the brushes giving a lower RPM, yet, a higher torque with the same voltage which may be what you are truly looking for. However, you need to observe the motor's idle draw current when doing this. Lowering the RPM too low like this will cause a gain in current draw eventually to a point where you will burn out the motor. (This means rotating the brushes axially (not turning the brush in it's socket) or rotating the outer electromagnets Mechanically, most motors don't allow this, for the old small circular DC motors used to almost allow it by rotating the rear cap of the motor which held the brush orientation, there was usually a peg or screws holding the motor's back plate in the optimum brush orientation.)