OK, the speed and gear ratio issues have been pretty well covered, I'll touch on the radio question..
There are several companies across the U.S. that advertize the conversion of old 6v radios from AM to FM and CD.. In all cases the conversion is very expensive and in my opinion over-kill for what you get..
I know several guys that have come up with a very simple solutions to the old 6v AM radio problem.. Eliminate the old radio box up under the dash, retain the original radio head and cables in the dash.. Purchase a inexpensive manual control AM/FM/CD radio, not one of the high end digital models... Mount the radio up under the dash, in approximately the original position.. Remove the knobs from the radio, modify the shafts so that the original shafts from the head will attach to the radio. Run the power supply from the original head to the new radio. Syncronize the dials/cables together.. The original radio controls the new radio, inserting a CD may be a little difficult depending on how the radio is mounted..
The whole conversion can be done for under a $100. and a few hours work...