Electrify a speedometer? Remove the speedometer cable drive. Cut the drive about 1 1/4 inches long and solder into speedometer drive. Attach the ignition positive to the shortened drive that is now soldered into it original location less the outer casing. Ground through the chassis. Sorry, I couldn't help myself.
The easiest would be to find an electronic unit of equal sweep as cable unit. Then it's a matter of transferring guts. Sounds simple but understanding and patients is a must. There are gauge repair shops that do these types of mod's.
I think some of the well known guage manufacturers do that as well as standard guages. Depends on how stock you want the finished guages to look.