You want to Gordon Ramsey them!
Put a suit and tie on and approach them. Try not to insult them and see if you could take over the kitchen, menu and pick your own manager to run the floor.
And I would tell them you will do it for free on the stipulation that if you do turn it around, you get a percentage of the take from the kitchen.
And remember this, you may be walking into a situation where the owner(s) know nothing of how a restaurant should be run. One of those individuals, or a couple, that always dreamed of owning a restaurant that have no business being in the business. If that is the case, tread carefully.