I pulled out one of my Rubicon Models SdKfz 222 kits as well as a spare 223 turret to have a hands on look. The bad news is that there is no easy fix other than if you have a spare MG. You can simply snip off the existing MG and prepare the area to take a new barrel after everything is painted.
Using Magnets:
EWG makes a good point about removing the tabs. However, to keep the turret from falling off I would use a magnet. This will require some work after you remove the turret tabs. Glue a magnet at the bottom of the turret (center). Then put a support structure or something inside the hull to hold a piece of metal. The important thing is to carefully make sure the gap between the metal and magnet is either perfect, or just a hair gap between.
Another idea is to glue the metal at the bottom of the hull and glue a plastic rod with a magnet at the end to the bottom of the turret.
I do not recommend using two magnets. With neodymium magnets, one magnet and a piece of good metal is more than what you'll need. Also you do not have to worry about getting the polarity wrong.