well there is a bit to installing a fuel level guage, let alone just a low fuel level indicator.
i wont put it inline as it is a LARGE picture, but if you look at
here at a picture of a car fuel level & low fuel light sender there's a bit to install. The right hand gizmo is the low fuel indicator which is a coil in a tube, depending on the fuel level the coil is allowed to heat up, or not. If it isn't covered up then it heats up (but its in a flameproof container, so thats cool) and lowers its resistance, allowing the light to illuminate on the dash. The lefthand gizmo is the fuel level float, controlling a armature connected to a high resistance coil (same kinda deal as in the oldschool scalectrix controllers). Either way you have to find a sender from a bike that'll fit as well as find a suitable spot on your bike (you'll have to cut a hole for the indicator gizmo to go through, as well as find a way to mount the gizmo securely (not secure = fuel on a hot engine, and that is veeeery bad!) like getting a couple of nuts tack welded into the inside to mount against (of course the tank would have to be aired for a LONG time before this goes ahead, plus scrubbed like hell to remove any petrol or any chemical within there that may ignite)
Shitload of work, technically possible, but the question remains, could you really be arsed putting the effort in just for a dinky light unless it was already designed into it in the first place?
p.s. i've had a bike with a fuel level indicator, current bike has only got a low fuel light. Personally as much as i'd LOVE a level indicator, i really couldnt be arsed sorting one, let alone figuring htf to mount it on the dash.