i may have some 3mm i may try i got em out of some led lite jewelry opps i have some in another led i have its blue but it runs on 6 volts guess ill need to reduce the 12 down?
The 3mm LED fits right in the top without needing to modify the reflector- and if you seat it right- you'll actually get focus like it did with the old lamp!
(I've done a few of these myself-)
Make a spacer for the battery with a dowel pin about the diameter of the battery- then add a 100 ohm or so resistor to limit the juice to the led by grinding a groove in the dowel and wrapping the leads around (end to end) the dowel.