Hi there the service that A Harper offers I believe is a push button for 12 volt then when up to speed there is another button to push it up to 24 volts .(This is what I have been told but I have never seen one ) but in all cases the standard gear box was not designed for it and at some point it could disintegrate unless you look after it and treat it well ..(no thrashing )
I do have a 24volt one but the diffence is you control it with a throttle cable ..not a push button