Looking at the original specs the car has a .12 airplane type engine (pretty normal in those days). I can not find a list of the K glow plugs but I think the 7 is rated too cold for such a small engine. With a low combustion power that this old engine generates it is difficult to heat up the plug to activate the next combustion and so it fades out and stops the engine. Beside that, the heat number of the plug also determines the ignition timing which will be far off...

I think you need to use a warm glow plug so I suggest to start with the 4.