integrate the internals of the demolisher into the deploy
Okay, so what you're best off doing here is essentially to use the magazine well of the Deploy, and the front end, stop the pump grip from moving, and use the back half (the pusher and the rev trigger) of the Demolisher in place of the stock. You'll be taking the dart tooth out and putting the flywheel cage where that was, as you don't have a bolt anymore.
That's my best advice. You'd lose the sliding gimmick, but keep the terrible flashlight and the re-orientable magazine well.
However! An even better idea would be to do the same with a Stryfe instead of a Demolisher, and give it a full auto kit, since Stryfes aren't expensive and are much less bulky.