ACTion's P13 is pretty damn loud and has adjustable tone, but don't skimp on a little speaker and/or low-voltage. You'll need 12v and at least a 4" x 8 Ohm x 10W speaker, mounted on a baffle board (6mm ply) with a good area of outlet for the air coming from the front of the speaker. You can trigger it from a non-proportional channel with a P43 Switcher or, if you're still fond of the 20th century, a microswitch on top of a servo (ugh!!). P13 Foghorn
If that isn't loud enough they do a 6W and a 15W amplifier, either of which will blow your wig off at twenty paces. BTW don't bother about an engine sound - you'd never have heard it unless you were in the engine room.