To use a non-12 volt LED would require some calculation to be certain that the proper voltage drop occurs across the resistor. Space will be the problem with the LED and the resistor. The "grain of wheat" bulbs are smaller than the smallest LED you're likely to find and will not require any limiting resistors
Checking their online catalog, it appears as if they may no longer carry those lamps. Here'* a link to the list of bulbs. Figure 12 seems to be your best bet.
A couple of other links from hobby sites that carry the bulbs. They are widely used, so a Google or other search should come up with plent of suppliers.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.