All you will need is the flashlight, a LED and a resistor or 2 LEDS and 2 resistors for the 2 LED version.
Here is a list of resistors and the currents I measured in the series circuit with a new 6 volt lantern battery, a resistor and a LED. The 6 volt battery measured 6.28 volts with no load:
150 ohm = 20.3 milliamps
168 ohm = 18.5 milliamps
220 ohm = 14.5 milliamps
300 ohm = 9.8 milliamps
390 ohm = 8.5 milliamps
The 168 ohm resistor is actually a 150 ohm and 18 ohm in series. This is the resistance I used for each of the two LEDS I used in the test setup for the 11 day test. The currents were calculated by measuring the voltage across the resistor and dividing this by the resistor value (ohms not cents).
There are different ways to do the actual modification. For mine, I removed the light bulb and broke the glass with a pair of pliers (do this out side over a trash can. Wear safety goggles. Yada yada, safety first , yada yada, adult supervision if under 18, bla bla). Then I cleaned the inside of the metal can with a dremel.
I have shown two versions re-using the original bulb. The tall one was used in my flashlight because it was not necessary to utilize the parabolic reflector due to the fact that I used the 15 - 30 degree angle type LED and most of the light can never hit the reflector. You can also make the stubby type to get the full benefit of the reflector but it is a little more work to get the resistor wired up down in the can. A two led version would be a real challenge. By the way, to make the connection to the base of the lamp you need to remove the solder from the base. Solder wick works well for this. With the solder removed you will have a hole in the base.
You can add wires to the LED and resistor and solder directly to the bulb contacts instead of upgrading the actual bulb. If you choose to do this you will need to remove the spring that is used to make contact with the bulb.