If you can't find a suitable cap made for a tractor, you might try adapting a generic indicating cap sold for riding mowers. They're sold at Lowes and other such places . You also might be able to adapt the float and level indicating gauge from an outboard motor gas tank. The assembly from the outboard tank wouldn't have to be part of the cap , but could be mounted anywhere on the top of the tank. Of course, cutting holes into gas tanks is not an easy task, due to the explosion danger involved. Even tanks that have been removed and cleaned can still have explosive mixtures lurking inside the tank. Using the modified gas cap method might actually be easier. Indicating caps are also sold for portable gasoline powered generators , although they have threads about the same size as the riding mower caps.