2x pulley's, 2x sheaves (Pump is 24mm on the GP I am pretty sure), 1 set of pump rails, and belts preferably BX belts and 2 of these.
Take gear box off pump and engine, turn one 90 degree counter clock wise, other 90 degrees clock wise. Figure out the the pump rail situation so that you can line everything up straight and tight.
Now in saying all this, if the gearbox has no mechanical issues keep it on and keep washing. The RPM's will not change, and the only other difference is that belt drive can typically handle the pulses and is easier on the pump/engine. Although considering how cheap either are to replace I would just replace things when they and or if they break as you will spend more money and time on the conversion then what it is worth.