Alot of times they are just optical sensor that operate on an on off scenario. The sensors seeing a different wave when submerged vs not. Key is to make sure the submergable sensor won’t be degraded by fuel. So when the fuel level is below the sensor, it turns on a dummy light.
https://www.bulkreefsupply.com/os-1m-optical-level-sensor-with-mount-neptune-systems.html?gclid=CjwKCAjwk7rmBRAaEiwAhDGhxNX7dagx0ZAsS113iP0K9t3ecwlQ8bkPPYnZv1aagQzlhvZEPwjZCBoCDnwQAvD_BwE Just drill a hole, seal the sensor...bing o bang o, you have mission complete light :-p