Rockola 1454 pre amp

Hi I am wantingg to fit a magnetic cartridge to my Rockola 1454, does anyone know the best an easiest way?
I can buy an external pre amp and power supply for £20 but would ideally like to use the Rockola power, this would enable me to use the single socket
Not being that confident with amp wiring I would prefer not to strip the valve amp down
Any assistance would be greatly appreciated
Thanks Andy