The easy way is to put the batteries where the tank and engine normally go, with a pusher motor at the rear and the radio in between. No need to interrupt the spar. A heavy enough battery would make it work fine. With a single fin used as a handle, launching by the pilot would still be possible. Since a tractor prop is destabilizing and a pusher prop is stabilizing, the fin could be made a bit smaller as well. Just another idea that I had.