Amp's not the problem, the power supply would be //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gifAsk JimJ if he could find you a 20w tube amp... they are fairly small that's about the best thing I can think of to fit inside an R/C car.
Unless you built like a little trailer to pull behind the R/C car. And mount the tube / amp and like a batcap or something back there. Good idea for a project tho.