ORIGINAL: motionmachine
I toy'd with the idea of using a stihl 09 chainsaw engine several years back. I found the magneto (coil) hung below the engine (crankcase) so the only real way I saw to install it was to lay the motor down or relocate the magneto (coil).
I guess relocating it would probably be best, for the sake of maintaining easy access to the plug. Of course, you would have to custom fabricate rigid and sturdy brackets to hold the magneto in its new position.
As for laying the engine horizontally, there are RCs that have the engine laid down in such a manner, some FG on-roaders do just this.
And if you are custom building your own RC from scratch, you could of course design it for easy access to get to both the plug and the coil.