The Marlin lever guns do have a tendency to loosen certain screws; mainly the hammer screw, and the finger lever screw. A dab of Loktite fixes this just fine. You don't even have to take the screws all the way out. Just back'em out about halfway, put a drop of the Loktite in the exposed threads in the hole on the right side of the action, and tighten them back up, Wipe off any excess, and you're good to go.
Re the crack in the stock, I get the impression you're saying the crack is in the tenon of the stock, where it doesn't show unless you take the stock out of the action. If this is so, and I've had it happen to me, what you want to do is keep the crack from spreading to the exposed area of the stock. I drill a small hole right at the end of the crack, then file a shallow channel right along the length of the crack and fill the hole and the channel with fiberglass bedding compound. This almost always "deactivates" the crack, and keeps it from going any further.