The Peter Spoerer "LocoSaver" level sensor is now available from Fosworks whilst the very simple G1MRA 2-LED version is described in their Newsletter & Journal No. 206. Both of these need a bushing in the boiler, but Fosworks do offer a solder-free bushing to make retro-fitting easier. This thread describes an optical type that uses the existing gauge glass.
I haven't used any of these, so cannot recommend a particular one.
Brian