If you want scale sound, well, that's a not too easy to do.
I've seen a large scale BF109 in germany with engine sound delivered through a speaker at the bottom of the fuselage. One of the problems is the models own engine disturbing the sound coming out of the speaker. Of course there are CD's out there with the correct sound for your type of model, if you're lucky it exists for different RPM-settings.
I'm an engineer in the "digital audio effects" field and from my view, almost everything is possible, you only need knowledge, time and (of course) some money.
One of the biggest and most audible problems is the so called Doppler-effect: This effect raises and lowers the pitch of a sound according to the relative speed of the sound source and the listener. (policecar siren approaching you seems to be of higher freq. than a siren moving away from you).
Our models are moving with much lower speeds than the real-ones, the effect with a rc-model is therefore much less, this makes your "engine" sound flat and a bit boring...
Conclusions:[ul][*] it can be done[*] it can be done very well, but you can't change the laws of physics (Doppler)[*] not shure whether the result is worth the effort (just my view)
[/ul]
hope this helps

Flo