From a quick look-see you appear to have a 10V stabiliser same as early Land Rover with Smiths gauges. There's an electronic one here:
Electronic instrument voltage stabiliser – 10 volt for classic cars (as BR13xx) | eBay
or you can go to a Jaycar store and buy a #7810 3 pin regulator IC for around $1 and mount it to a suitable small heat sink and solder some leads to it.
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