I wouldn't mount it on the engine. The vibrations and heat wouldn't be good for it. I like the seat tube, or under the engine on the down tube. The problem is the short spark plug lead the factories give you. Unless you replace that wire you're restricted to where you can mount the CDI.
on all premium builds, I mount either behind the air filter or under the engine for a nice clean look - this requires a longer spark wire & for under the engine, requires heat/chemical resistant wiring (I also make a small terminal strip for the connections)