The KF radios all used a 4 ohm speaker. It would be best to find a 4 ohm, however, with vacuum tube circuits it is not that critical that it be exactly 4 ohms. If you used an 8 ohm it would work OK but the output tubes may wear out a little sooner than they would using a 4 ohm. The original ones were 4 ohm, 9 inch oval, but a round one would work OK, it's just a matter of finding a good way to mount it. Voltage is not a factor, ohm rating and watt rating is and since car radios of the day were very low watt output that really won't matter either.