You may want to check your documents and see if there is any specs for your monitor. If so it may have wiring diagram and possibly info on the LEDs.
If no info, then you either see if you can find info online.
The other option is to go from scratch and build your own monitor. (Radio Shack or other electronic part supplier such as Auto Zone, Digakey (spelling), McMaster-Carr, etc.)
Pull the monitor from wall and look at wiring. Go larger gauge if you can't guess what is right. Check the voltage of the wire you will tap into and try to match your LED to it. LEDs are very sensitive to voltage. They come in all kinds of voltages. If I remember correctly you can input resistors into the circuit/circuits to help drop voltage to LED.
Mainah I'm sure will be able to be of more help.
What I like about my 1983 T-1550 is that the tank is right by the door under the stove. In addition it has a door that pulls open (pivots down) allowing me to see the port hose line in as well as the tank, thus just watch it fill.
My 1996 T-2053 also has the fill port next to the door under the sink. This has a panel with four screws. If I take this panel off (and I do) I also watch as it fills. I may convert this into a door much like the T-1550 as it is a snap to just pull door open and view. Neither the 1550 nor the 2053 have any electronic monitors, just little old me, which is just fine. One less thing to go wrong. I can do that all by myself!