Assuming that yours doesn't have an armoured pump i.e. you can get to the stop solonoid, just run a lead from its terminal to a switched ignition supply, that is if it turns but doesn't start.

If the starter is not turning its possible that the ignition switch is faulty, rather than change tthe switch you could 'modernise' it by fitting a push button switch between battery+ and the small connection on the starter solonoid.

If it starts when you short the two heavy terminals on the starter together but not when you apply 12v to the solonoid small terminal, you starter solonoid is faulty.