I suppose we have to define "qualified gunsmith". There are a million guys out there, (well at least several thousand) guys that replace parts and can maybe even mount a scope with a gunsmith shingle hanging, and a business card, that could easily make that problem worse.
I presume you mean the stock screw is loose. Several things could be the cause. Generally Marlin stocks were fit tighter than a bulls butt in fly time, but it may have been replaced, or it got loose 50 years back, and the recoil then caused the hole to enlarge and wallered out the hole, thus no amount of tightening the screw will do anything more than "fix it" for a bit.
Lots of guys plug the hole, then drill a new one, but to get that new hole properly located, is more difficult than it looks. And then the hole needs to be a near press fit, or a two step hole on the screw diameter to really be correct, and not loosen up first trip to the range.
Shipping guns at present, can be a headache and expensive at that. I am sure several guys here know someone that could do the work, given it could be shipped in a simple manner. As far as I know, a gun can still be shipped to a gunsmith, and returned to same owner, but dont quote me on that. But then Fed Ex or UPS has its own hoops to jump through.
In any case, the job isnt rocket science, but you need to find a guy that has worked on such jobs before. Do not take it to your local 1911 expert, and expect him to really savvy the correct repair. He might or might not know. It would depend on his background in guns.
A gun of that age isn't considered a firearm in the legal sense. You can just pack it up & ship it at will without telling them its a gun. But even if you did tell them as long as its going to a smith with an FFL its no biggie.
I just went hunting in Oklahoma, I shipped 2 modern Marlins to myself CO a hunter in OK & then shipped them back home. I had to explain the laws to Fed ex on the way back but it was no biggie. On the way out they thought they could make their own laws so I used UPS who listened. The BATFE has a website with the relevant laws. I'd suggest you print them out to show the shipping co of your choice to avoid confusion. Or, since its an antique just dont say anything.
I remove the butstock anytime I ship a levergun now because I had one busted. Shouldn't cost you more than $25 or so.
As far as a smith to work on it I'd feel safe having my guy look at it & offer suggestions as to a fix. But I can just drive there & talk in person. I dont think I'd send an original to a stranger.
Is it the tang screw & butstock loose or the forend?
If the screw is loose in the tang, then the threads on either the screw or the tang are worn out. A new screw might tighten it up, but if it doesn't then you'll need to do one of two options; have the hole welded and re-tap to fit, or have an oversized screw made and tap the hole out larger to fit that. I've done both ways, but prefer to weld the holes and re-tap for original size screws.
John Taylor Machine can easily accomplish the task, if you don't mind shipping the gun. http://www.johntaylormachine.com/