Shouldn't be a big deal at all. You could even go with a wing off to install packs if need be, install a kill switch outside the Cub to safe the system.
Otherwise wouldn't be that hard to install a hatch in the belly.
Your motor/esc combo would work well. On 4 cells and a 12x8 prop, would pull approx 39 amps, deliver over 4 1/2 pounds of thrust and 60mph pitch speed, which would be great for scale flying and even a little more.
Go for it!