Well, just responding to your first part saying the receiver needs to put out 12 volts. That's a problem considering you should only be putting in ~5 volts, I'm not sure if you know this or not.
But as people have been saying, you can just hook a 12 volt battery up to a remote switching system and then wire the PWM of the switch to the receiver.
People are suggesting standard FIRST relays, I would suggest a Team Delta RCE210 Switch (
http://www.robotcombat.com/marketpla...mdelta_rc.html, scroll down to see it.) It is around the same price as a spike, it can handle 24 amps which is 4 more then a spike and go up to 30 volts as where a spike can only go to 12 volts. You many not need 30 volts, but if you ever want to upgrade you don't have to buy a new system.
As far as a radio, without knowing what FM radio you have now I can't really tell you if you should upgrade or not. If you didn't have a radio and I was recommending one, I would say the Spektrum 6 Channel system.(
http://www.robotcombat.com/marketplace_rc_radioDX6.html) It is a standard 6 channel aircraft style remote on the 2.4ghz frequency so there is no interference. The only reason I linked you to the Robot Marketplace again is because they have the specially designed BR6000 receiver there. The original receiver doesn't have a failsafe so when you loose signal the robot keeps going. This is a big problem in combat robots (another thing I do) so Spektrum designed a special robot receiver with a failsafe. I would definitely suggest that you get this considering that if your project goes awry you would have a big problem.
-Greg