Ted, the speaker can go in the tank, you won't be using the battery any more. You'll just have to cover the top of the tank hole with a platform, 1/16" Styrene is a good choice.
The real question is if there's room between the horizontal motors for the electronics. Obviously, the battery box, horn relay, E-Unit, and horn will all be gone for the upgrade.
Interesting that both of the A-A units are powered, must have sucked a lot of power!