^ Seems like something good to do to the plastic encapsulated type with the rounded focal dome on top. For those, I take a file or sandpaper on a flat surface and completely sand off the dome to a flat top then polish to clear again, but for the surface mount LEDs none of this is needed, though then you need the PCB to solder them to or if star heatsink mounted, still a more elaborate mount (maybe, depends on the structure behind the area needing illumination).
The "best" way to do it all depends on the desired outcome. If looking to restore to original state then you should use same bulbs and it's easiest because the design is build around that. If looking for extreme bulb life (why? Surely that old radio can't be such a pleasure to listen to that you have it running all day long?) then you can tap off any lower voltage circuit, rectify and smooth and current limit LEDs. You could even get one of those long, thin COB style so you have less of a burden spreading the light and mounting it, though most of those put several LEDs in series so need a higher input voltage.
