I have only a very poor image of the sender, I was leaned over a running engine I should have checked it & realised how poor it was.
This is the panel that is in the boat, but we're not sure if it has ever worked with this engine as the anecdotal history says the engine was replaced with a recondition one around 5 years ago, but we don't know if it was of the same type or not.
But nothing works on the panel apart from the stater button & the ignition warning light which is just floating in the back of the panel.
I think as suggested the obvious answer is to remove the existing sender & gauge, by a matched set & get a convertor to go from the large current thread to a more usual one, somebody has suggested it might be 5/8th UNF is that likely?