Druid, did that saw come with a motor at all? If it did, and you knew its rotational speed, you could go by its pulley size to determine the new pulley you'd have to use. For instance, if the old one was 1750(1800 rpm), with a 2" pulley, you'd want to go with a 1" pulley on the 900rpm motor. This gives you the same speed on the blade.
For what it's worth, I don't think you'd have any problem with a 2kw motor having enough torque to be able to under-pulley it.
Motor speed is not nearly as important as torque, as if there is enough torque, you can run multiple combinations of pulleys to achieve the blade speed you want.