I’d recommend the Redline N7. Since it seems you are wanting to ditch the tank with CO2, ditch a battery too and get the N7 Milsim. The only sacrifice you are making would be full auto, but you could also spam the trigger once you have it tuned well. It’s a fairly inexpensive engine, simple to install and is no fuss. I love mine and am anxiously waiting for spring to come so I can get out and do some work with it.