If you still have a dynamo it should be easy to switch to the battery negative earth norm. You then have a greater range of solid state instrument regulators. I used this one on a Land Rover.

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Note the one you originally ordered appeared to be electro mechanical, likely as below.

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Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone