According to the location and shape of the tank, many fuel level sensors are possible. Simplest is to put a tube attached top and bottom of tank side and view fuel level in the sight glass. There are also ultrasonic fuel sensors that operate almost like a fish finder to determine depth of liquid in the tank. The most traditional is a float in the tank on an arm that pivots on a potentiometer that sends a signal to the gauge. They require an opening in the top of the tank to insert the float, lever, and potentiometer. You can find many aftermarket gauges by doing a search on Google.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI